changelog: update for version 3.3.3

migrations: add missing migrations

rendering: really fix the no slide issue

assets: fix storage of assets

CHANGELOG.md: update log

CHANGELOG.md: update description

CHANGELOG.md: adjust titles

CHANGELOG.md: fix typo

api: update for review changes

api: update out of range check for meeting name and meeting id

api: fix missing POST headers if an asset is sent