Add support for VTT or other subtitle formats on ingestion + display

[bctcvai]

Tator would benefit from utilizing something like VTT for its text overlay capabilities. Currently a relatively static menu is available for overlaying camera numbers or calculated date of time based on filename.

Related issues include #1501, #1133, #407

There are a couple of thoughts here:

• Ingestion of VTT files (coupled with conversion of other formats such as substation alpha)
• Display attached VTT files "as-is" bypassing anything particularly fancy
• By able to dynamically calculate VTT files based on user preferences and display those.
  • A flexible way to handle this might be utilizing an applet-like approach. A VTTApplet type could be responsible for generating the VTT file supplied to the video player.