Better off topic tags

[minalike]

Closes #2018

As discussed in staff channels, we are modifying the original !ot embed to only list #ot2, since the tag's primary use is to redirect users to an off-topic channel. By only listing #ot2 the hope is that there will be less interruptions to the very traffic-heavy #ot0 channel.

A new tag called !off-topic-names is introduced to serve the content of the original tag. Channels are listed in the original order, with a brief explanation of the nightly name changes, and a link to the off-topic etiquette page.

[minalike]

to quote kat:

I think it's shown a beneficial change, even from the perspective of people new to our server and off topics. Rather than posting in a busy channel and getting quickly drowned out, their message is now given more space and isn't likely to interrupt and ongoing convo.
Looking at percentages of where users will post first messages in off-topic:
ot0: 76% -> 71%
ot1: 13.5% -> 14.22%
ot2: 10.22% -> 14.67%
ot2 even had more messages posted it than ot1 for this latest stat pull. That indicates to me that the embed is successfully redirecting folks. So why not just clarify the embed further, only link ot2, and take our improvements for a marginal change.

[janine9vn]

Looks good! I would suggest committing Dawn's suggestion because I like the wording of "often", but it's not a blocker so I'm going to go ahead an approve this.

Otherwise, as a note, this does mean that !off-topic will fuzzy-match to the longer embed and not !ot, which may at first seem like inconsistent behavior. But actually, no one really uses the full !off-topic tag invocation. The usage for the past month is almost entirely !ot, so I'm fully onboard with changing the tag name to !ot.

I would like to add an issue for adding an alt-names key to tag metadata, but that's not required for this PR.