Add ability to specify additional work tags

[irrationalpie7]

Some people like to use an audio format tag like "Audio Format: MP3". Technically this could be done as another dropdown like the length one, but if we consider only canonical audio format tags we won't include "Audio Format: Download", which at 590 uses is more widely used than the canonical tag "Audio Format: M4A" ‎(325). Also, it's a little trickier because e.g. "Audio Format: Streaming" and "Audio Format: MP3" aren't mutually exclusive, so a lot of people will use both (14061 works as of right now, apparently.

Technically the audio format tags were the only ones specifically requested by the extension users I talked to, beyond a misunderstanding about the "Podfic & Podficced Works" tag, but I'm also wondering if simply allowing a free-form comma-separated list of custom tags would also help for the non-podfic use case, where someone wants to make filling in metadata easier by pulling from a similar work, but might be posting fanfiction or fanart rather than a podfic. They wouldn't need the "Podfic" or "Podfic Length: X" tags, but might want something else instead, like "Fanart".

[jermowery]

WRT free form tags I think we shouldn't support that particular part of this FR. The page the user is on, the AO3 New Work page already has free form tag editing with autocomplete and chips, I don't think we can do better than that.

As for the audio format tag, we could add a new select or checkbox for that

[irrationalpie7]

Got another request for this (specially re: audio format tags). I vote for scoping this issue to canonical audio format tags + "Audio Format: Download". Whatever we choose would ideally allow people to select multiple audio formats if multiple apply

[irrationalpie7]

For reference, here's the full list of audio tags, including non-canonical tags. I definitely don't think we should support all of them, but that's where I got the info on usage of audio format: download