[Accessibility] Providing labels for alternative content

[hostle83]

Describe the bug

Transcripts and other alternative content can be uploaded, but is not clearly labeled as such.
For example, if someone uploads a video (with audio), s/he can upload a transcript, but there is no label for "Transcript" or "Alternative Content".

Expected behavior

Create a label/field for alternative content. Ideally, prompt users to upload alternative content.

Related PR

#3807