Allow filtering by multiple users

[TheAMM]

⚠️ Merge after #592 ⚠️

...by repeating &u=user in the GET parameters.
No UX for this yet.
Reworks the RSS URL generator to fit with duplicated keys.

Sort of tested but could use a sharp look at.

same_user = False
if logged_in_user:
    same_user = len(user_ids) == 1 and logged_in_user.id in user_ids

should replicate the previous functionality well enough.

[Vodes]

Is this ever going to be a thing? I'd rather not get ratelimited trying to avoid trolls ^^