Use slash commands instead of message listeners for stored replies

[vcarl]

Alternative approach to #148, will close #131

This puts every !command under a single top-level /r command, with an autocompleted list of replies as the first argument. It includes the idea introduced by #240 (thanks @ScriptyChris!) of telling the bot who to ping — #240 had the problem of double-pinging, but as a slash command argument it can be done without that drawback.

This code removes some commands for simplicity, it's a functional proof of concept more than final code. Remove commands:

• !mdn
• !lock
• !unlock

Another change: !commands has been changed to /r help, which uses an ephemeral reply to avoid shoving everything off scroll back.

This is very much a rough draft, functional but incomplete. Before merging we should add logic to encourage folks to shift from !commands syntax to slash commands and figure out how to handle !mdn

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[ScriptyChris]

Shouldn't this long commented section be removed (i guess it's a remnant from code testing)?

[vcarl]

Yeah this is more of a draft state atm, roughed out. I'll update description to add necessary followup before this can merge