Hs/large attachments - Fixes #109

Partial fix #89: Disable room ping @here, @everyone

Fix puppeting (get_user_discord_ids in store.ts), fix tools/addbot.ts

Broader emoji support

Move Mention parser to MatrixEventProcessor

Make sure usernames don't overflow the length requirement

Use a thumbnailed image for sender avatars

Discord uses 128px square images, so 512px should be fine for most cases.

fix discord message content parsing

Fix checkboxes on README, check Webhooks

Change bot starting command to npm start

Helps to differentiate the start command and the command that generates
the config. I believe `npm start` also makes sure the code is up to
date.

fix html escaping, #94

* Check WebHooks
* Fix spacing of checkboxes so they render correctly on GitHub

Self-service bridging (for existing matrix rooms)