Rename pending request channels with amount of pending requests

[violine1101]

It would be great if the pending request channels would be renamed so that the amount of unresolved requests is immediately visible without the need to visit the channel. (e.g. #20-java-pending-requests)

This may be a bit difficult to realize, not only because the bot needs to know how many unresolved requests there are left (which means it also needs to track message deletions etc.) but also because there's a rate limit on how often discord bots can edit channel titles. I'm not entirely sure as for what the exact rate limits are, but they exist (the Minecraft commands discord has had trouble with the channel renaming rate limit, but they also use it very extensively).

[violine1101]

To clarify, I'm talking about our internal channels here that aren't visible to the public. The reactions in the public channel don't really have much to do with this, and keeping track of unsolved requests that way would probably be very inefficient.

But thinking about it, we know how many messages there are in the internal channels, so that value could probably be used.

[violine1101]

Some considerations from #136:

• The rate limit for channel name and topic updates is 2 updates per 10 minutes per channel. If we exceed that, the bot will get rate limited.
• The amount of messages in a channel should always be known because all messages are loaded on startup. Still it might make more sense to have a global count of pending requests for each channel and update that as requests are opened/closed, in case the cache is invalidated for some reason.
• It might make sense to add the amount of pending requests to public request channels too, although perhaps in the channel topic instead of the channel name.