Improve bot challenge logic

[lucasart]

Current logic is Play -> Online bots, then:

1. select from a list of all bots connected. This list is too long, and sorted in a unintuitive way: features bots, followed by others by descending time spent playing.
2. select time control, variant, etc. and challenge

I propose to invert the logic:

1. first select time control, variant, etc. and also elo range (relative to user)
2. then display a list of relevant bots

This seemingly trivial inversion has many benefits IMO.

• rating filtering For example, user selects 10+5 rated, with -300/+200 elo range. 10+5 is Rapid, so (2) should only display bots whose rapid rating is in [X-300,X+200], where X is the rapid rating of the user.
• reliably rated bots only Use the max rd value corresponding to lichess.org showing '?' in front of ratings (unreliable). For example, if the user selects 20+10 which is Classical, very few bots have rd <= 100 (or whatever the value is) in Classical, because they don't play it, so the list should be reduced accordingly. Even more pertinent in Chess960, or any other variant, where the vast majority of bots do not play and have an rd > 100. This filter could be optional in (1), to cater for the special snowflakes that never play rated games (including Leela piece odds bots).
• improved sorting given the context provided by (1), you know which rating should be displayed, and used as sorting criteria for (2).