Request for a new API: Update captions by transcription ID

The current caption updating API in `/api/Captions` can only update a single caption line with one API call, which is insufficient for frontend issue [#103](https://github.com/classtranscribe/FrontEnd/issues/103) for two reasons.

1. Bulk editing of captions needs to update multiple caption lines (text, start time and end time) by transcription ID, with a single API call.
2. User should be able to upload new caption document by transcription ID.

Therefore, it would be great to have a new API that can do PUT requests by transcription ID, to update/create captions efficiently.

Specification:
- The API should take a transcription ID, an index of caption array in the transcription array (for instance, currently, index 0 is for English caption array in a transcription array), and an array of caption object. In the frontend, the form of the caption objects in the caption array should be:
`
{
begin: "00:00:00.0400000",
end: "00:00:03.2400000",
index: 1,
id: "4d5a6020-2fb1-4dc3-93f8-4845bbc2c131",
text: "Reflecting on what makes an activity or hobby or what"
}
`

- Frontend will provide a complete array of caption object, including the objects that are not modified. Backend should modify the captions according to the input. For captions that are deleted, the index should be -1, and the backend should handle the change of index accordingly. For new insertion, an id for the caption line should be generated.

More spec will be added...