util: improve thread slug sanitization

invites: add QR code invite representation

add a qr code invite by using https://github.com/skip2/go-qrcode

the procedure is as follows

1. var invitePNG []byte = qrcode.Encode(<invite link>, qrcode.Medium, 256)
2. encode invitePNG as base64
2. use data URLs (data:image/png;base64,<data>) to encode png on page
3. add qr section to template html/admin-invites.html

limiter: wrap concurrent map access in RWMutex

limiter: add limitall to the important bit too ':)

limiter: add method SetLimitAllRoutes

use gomod redirect

Merge pull request #33 from cblgh/password-proof

add password reset functionality