I'm not dead --- code cleanup and really real ratelimit bucket(s)

[dlmarquis]

I had a ton of IRL things going on this summer and disappeared, but these are the odds and ends I've managed to find time for since Spring. I'm submitting this as a PR instead of pushing directly to master because it feels a bit rude to drop off the face of the Earth for months and then push several commits that rearrange a bunch of code without warning.

Items contained in this PR:

• Updated various structs and the capabilities string, to match what Discord have updated themselves.
• Moved a bunch of duplicated reply/react-related code into their own functions, mostly text formatting stuff.
• Also made a centralized display-name-fetcher function, because that code was duplicated all over the place as well.
• Created a token bucket struct and related functions, so we can do ratelimiting properly instead of adding magic-number delays and praying. So far I've only implemented an account-level bucket for gateway requests. Tripping that particular limit results in a 30+min IP ban and/or locking the account behind phone-number verification (ask me how I know), so if you want to run a stress-test on the bucket, I highly recommend doing it on a throwaway account at your local library.
• Fixed an issue where the guild member list handling would request to sync the next 100 members before receiving the last batch requested, if the gateway sent an update on already-synced members before processing the last request. If you still want to stress-test the above gateway bucket, you can revert this commit to help things along.

[alexolog]

Welcome back @dlmarquis!

Thank you for your work, it is much appreciated!

When you have the time and the inclination, can you please look at the following issues:

• Outgoing messages not sent
• Incoming messages do not arrive
• Conversations that happened on another client when Pidgin was not running are not fetched
• Duplicates of old (sometimes very old) messages recur. This often sometimes when Pidgin loses connectivity. Easy to simulate by turning the router off for 15min

Those, in my opinion, are the most significant issues as they affect the core functionality. There are multiple tickets logged about them (by me and by others).

[dlmarquis]

@alexolog Thanks! And I absolutely will.

The stuff in this PR that touches the guild member fetch and the gateway limiting was actually intended to fix a few causes of the incoming/outgoing messages problem. And I'd planned on the next phase of that work to be rearranging the discord_fetch_url* and related functions so that we could display useful errors to the user (e.g., "request timeout, message was not sent" or "You are being ratelimited!") to aid in troubleshooting the rest of them.

The conversations viewed elsewhere not being fetched is partly due to how we check for unread messages before we fully load in the buddy list. The buddy list is where we keep pidgin's "last seen message" for each channel. I've been meaning on rearranging the startup code for a while, and I did have a version where we loaded the buddy list before handing unreads in #376. Doing this will also let us streamline the way we handle those unread messages at startup.

For the message duplicates, I think I know exactly what's causing that. The way the different history fetching cases work is a bit of a convoluted mess, and the way they all connect into discord_join_chat and discord_open_chat needs to be gutted and rewritten (which is why it hasn't happened yet). Fixing and rearranging the startup code to fix the unreads problem is going to significantly impact how much work has to be done here, so this one should be tackled last.

[alexolog]

Thank you so much for working on Making Pidgin Great Again! I kid! I kid! But the appreciation is genuine!