Add markdown for tags

[cellio]

meta:288221

Request: markdown for tags:

• In a post: styled as if the tag were a tag on the post (including required/topic/mod-only styling), and links to that tag's page
• In a comment: no special styleing, and links to that tag's page

From elsewhere, people are used to [tag:name], and our own search syntax includes tag:name (no brackets). If we can do [tag:name], that's probably best for ease of adoption. If we can't do that, can we do some other shorthand syntax?

[MoshiKoi]

I have a proof of concept on https://github.com/MoshiKoi/qpixel/tree/tag-syntax-experiment, which uses [tag:name] syntax. It isn't able to include required/topic/mod-only styling though, because we don't expose that information through the tags API. Thus, this requires a change to the backend as well if we want that feature.

[cellio]

Approaching it incrementally sounds fine by me! If we can get the syntax and basic styling at all, we can make a follow-on issue to add the backend support for the enhanced styling.