Support for uploading subtitle (vtt/srt) files

Users submit a language and a label to be included with the subtitle. Includes tests.

Requires the `SUBTITLE_DIRECTORY` configuration path to be specified and Flask to have write access to it. This is where subtitles are placed when they are uploaded.

Subtitles are returned in the following format

```json
[{
  "@id": "http://example.com/some/path/file.vtt",
  "language": "en",
  "name": "English Track"
}]
```