Add an overlay for draft subtitles explaining their status

[percidae]

If a user chooses to display a draft subtitle on media, the player should display an overlayer explaining that this file is a draft and where it can be improved.
Showing this disclaimer for 30 seconds should be enough.

Example for German talks:
Dieser Untertitel ist noch nicht fertig.
Wenn du kannst, bitte unterstütze uns hier
und schau den Talk in Amara an für die
letzten Korrekturen:
https://c3subtitles.de/talk/1345 Danke!

Example for non German talks:
This subtitle is not finished yet. If you
are able to, please support us and watch
the talk in amara for the last changes:
https://c3subtitles.de/talk/1379 Thanks!