Add subtitles w/o admin

[MatthiasReumann]

Motivation and Context

• Add subtitles model and dao
• Add /api/stream/:id/subtitles/:lang endpoint + tests
• Dynamically load subtitles on the frontend

Steps for Testing

Insert subtitles to database for stream

• 2 Livestream(s) one with, one without subtitles

1. Navigate to the Livestream with subtitles
2. Subtitles should be activateable
3. Navigate to the livestream without subtitles
4. No subtitle button should be visible

Further Information

Not sure, but #891 might break the subtitles in split view. (@mono424)

[github-actions]

Your Testserver will be ready at https://898.test.live.mm.rbg.tum.de in a few minutes.

Logins

Kurs1	Kurs2	Kurs3	Kurs4
public	public	loggedin	enrolled
prof1	prof1	prof2	prof1 prof2
student1 student2 student3	student1 student2	student2 student3	student1 student2

[joschahenningsen]

Works great, thanks a ton! ❤️