Add an initial resync_versions API to v3

[ericholscher]

This will be used in the frontend,
but also available as an API.

Mostly curious if this is a good approach,
and I can get some tests together for it.

Looks like:

-> curl -X POST http://devthedocs.com/api/v3/projects/read-the-docs/sync_versions/ -H "Authorization: Token $TOKEN" 
{"triggered":true}

Refs #6090

[humitos]

I think this PR is fine. However, I'm proposing to use PATCH method over a URL we already have. This will simplify the PR and keep our endpoints semantically consistent across them.

Seems the implementation is hard.

[stsewd]

Code looks good, we should document this new endpoint.

[stsewd]

this should be below the next section...