Add field for specific sentence video to annotated sentences

[Jetske]

A text field must be added to the sentence upload and edit pages. It can contain a url to a specific video (so it's something different than the general source).

Also add this in the API!

[Woseseltops]

And if something is added to the API, also add it to the API docs here: https://github.com/Signbank/Global-signbank/blob/master/docs/signbank-api.json

[susanodd]

@Jetske on which models does this have a migration?
(dictionary, video) ?

[Jetske]

@susanodd only dictionary probably