Handle null values for toTeamIds

When sending a broadcast (2023_06 spec), `to_team_id` is null,
and the code creates `[null]` for `to_team_ids`,
causing verification failure with:

Clarification to unknown team null
Clarification between teams

This is introduced from 39ac2647772a52aee5e3cce886a45b66bd7fa977

URL account tokens for reaction videos

Adds account-specific token parameters to reaction video downloads. Any client using
the url as-is (without basic auth) can download the file as the given user.
This allows (e.g.) web-based clients to pass the url to players like video.js that
support https but not basic auth.

The token is a one-way hash based on the account user and password. It is only
implemented for reaction videos for now to get some use and feedback, but could be
used for other 'file downloads' as well. Since all traffic is via https it could
probably use a simpler algorithm, but this can easily be changed later.

I moved the existing static method used to determine which account is making any
request to HttpHelper since it is not contest-specific, and added the new token
helpers there.

Example of reaction video in /submissions or event-feed after this change:
{"href":"contests/baku-live/submissions/1/reaction.webcam?token=DSjPuOj7OL9HLxEtMe0a5Mrl1vGCz2N2cO1bidDA=","filename":"reaction.webcam.m2ts","mime":"video/m2ts","tags":["webcam"]}],"contest_time":"0:07:47.704","time":"2025-05-26T09:55:53.702-04:00"}

Reaction video tagging

We record webcam and desktop for reactions videos, but there is only one reaction
property, so we've been serving desktop as one file and webcam as another, i.e.:

{"id":"2",... "reaction":[{"href":"contests/baku-live/submissions/2/reaction-webcam","filename":"reaction-webcam.m2ts","mime":"video/m2ts"},{"href":"contests/baku-live/submissions/2/reaction-desktop","filename":"reaction-desktop.m2ts","mime":"video/m2ts"}]}

The only way you could tell these part was based on the filename or url - but now the spec
has tags, which are a much better way to differentiate them.

This PR makes two changes: renaming the files to use '.' instead of '-' to match the variant
format in the draft spec, and adding support for these tags. The new output looks like:

{"id":"2",... "reaction":[{"href":"contests/baku-live/submissions/2/reaction.webcam","filename":"reaction.webcam.ts","mime":"video/mp2t","tags":["webcam"]},{"href":"contests/baku-live/submissions/2/reaction.desktop","filename":"reaction.desktop.ts","mime":"video/mp2t","tags":["desktop"]}]}

(file extension and mimetype already changed via PR #1236)

Add problem attachment support

Add attachments to problems as per https://github.com/icpc/ccs-specs/pull/226.

Improve mime type handling, especially for video

Fixes three things:

1. Move the existing utility functions out of DiskContestSource to create a reusable
   mime type utility class, to avoid some of the duplication and differences in
   behaviour.

2. The mime type wasn't being set when downloading reaction videos - but using the
   new util fixes this.

3. When we started supporting MPEG-TS there was conflicting information about the
   correct file extension and mime type, and based on google AI and video.js we got
   it wrong:

   MPEG Transport Stream (MPEG 2 transport stream) <-- this is what we actually support
   File extension: .ts
   Mime type: video/mp2t
   188-byte packets

   MPEG-2 Transport Stream <-- variant used by Blu-ray & AVCHD disks
   File extension: .m2ts
   Mime type: video/m2ts
   192-byte packets (4 bytes added to help file seeking)

   This switches us to use the file extension and mime type of the first.

Remove contest location docs (moved to spec)

Contest location was moved into the spec, we no longer need to document it.

Include spares when reseting contest floor origin

When a floor map is saved to disk it automatically moves the origin to top left,
just to pretty-print values and have 0,0 at the top left. Unfortunately it was
missing spares (empty desks) when doing this, so they were always 'off the map'.

Update file reference tags consistently

Found and fixed a couple more places where file reference tags weren't being updated,
and moved it all to one method in FileReference.

Light mode for organization logos

Part of #1228.

Light mode support for contest logo, banner

Another part of #1228.