Fixed multiple Issues

[ssrahn]

• Download link should initiate download #782
  • Using download attribute doesnt work
  • I implemented it with axios
• Order of elements in "Link Courses" dialog unintuitive #770
  • Rename top element to "Verknüpfte Veranstaltungen" instead of just "Veranstaltung"
• Deleting videos does not delete them, but moves them to a "deleted videos" list #764
  • Rename Action "Entfernen" to "Zum Löschen markieren"
  • Filled DeleteDialog with infos what happens with a marked Video
• Contradictory limitation of number of characters in title of playlists #748
  • Limit input length to 255 characters
• Add videos to playlists: Info message does not close #746
  • Messagebox is now closable
• Subtitle file upload leads to unknown error #744
  • I cannot reproduce error
  • Added a success Message
  • Improved styling of the form
  • Add some text telling which file format is supported
• Wording of server delete configuration #714
  • Rephrased server delete dialog
• Making sure the server id is a Number in globalOptions
  • There was a display bug on the Testserver Admin configs (Default server config)