Improve the `tag` command

[indiv0]

• PRIORITY: determine why creating a new tag via DM poisons the tag DB
• ensure the TAGS field doesn't get loaded lazily (possibly read it during command initialization?)
• perhaps get the filename to supply to tag::Config::new from the global config, instead of writing it directly?
• perhaps take references instead of Strings, etc. in the Tag::new constructor?
• handle this failure case better
• make the tag::Config::load function async (or call it async in Config::new)
• make the tag::Config::save function async (or call it async everywhere)
• return a list of suggestions from the Tags::get_tag method if the exact tag is not found
• ensure that locks are performed once per call and maintained, to avoid issues as described here and here
• ensure that this, this, this and this retrieves the GuildId even if it is not in the cache
• clean the tag content here and here
• perhaps make tag insertion/modification by editing the DB in-place here (or use put_tag instead?)