Add tag support

[dawnofmidnight]

(part of the Carl ⇒ Ferris migration)

We'd like support for tags in Ferris. These are, similar to Carl, pre-programmed messages that Ferris would send upon the invocation of a particular command in a channel. These should support at least the following commands:

• ?tag create <name> <contents>: create a new tag named <name>, errors if already exists
• ?tag edit <name> <contents>: change the tag named <name>, error if not found
• ?tag delete <name>: remove the tag named <name>, error if not found
• ?tag <name>: prints the contents of the tag (with allowed_mentions set to nothing, preferably)

Open questions:

• Do we want top level command access, i.e. ?code instead of [?/]tag code?
• Should we have a cooldown on specific tags? That is, if someone sends ?tag code, should we limit another user being able to send it in that channel for the next X seconds to make sure that there're not several invocations of the long embed in quick succession.
• How do we remove a useless tag invocation? Detect deletion of the invocation message (for non-slash commands)? Use a context menu only usable by the invoker? Have a "Delete" button/reaction limited to the invoker?
• Should any of these commands be restricted to moderators?
• Should they be programmable? Carl has special metavariables to reference the current channel or the user sending the message. Do we need this?

[ivandardi]

I am working on this, will push branch soon.

[troiganto]

I don't think we've ever had need for programmable tags. Cooldowns haven't really been necessary in the past, people are aware enough that spamming a tag tends to have moderation consequences.

User-based restrictions have been relevant in the past, so we probably want the ability to flip these:

• limiting tag management to mods,
• banning individual users from creating/deleting tags,
• locking individual tags.

There's a lot of design space in this and I know the Carl devs put a lot of work in, so mirroring this feature likely won't be easy.