Timelines and mentions should only take user IDs, not usernames

[SamHames]

I think these two endpoints should only take user IDs, and not implicitly perform the lookup if a username is passed.

This would let us:

• avoid the complexity of dealing with usernames that might also user ids (I think the current implementation has some weird edge cases - a username that doesn't exist will raise an error, but a user id that doesn't exist won't)
• align the functionality with the Twitter API at the client, rather than doing an unexpected username -> user_id lookup

We'd probably need some supporting documentation to show how to convert usernames into user ids too?

[igorbrigadir]

How about a twarc2 usernames input.txt output.txt command? Something like that?

It's convenient to use usernames - maybe we can still do this but only if explicitly set, to avoid checking?