Switch markdown from blackfriday to goldmark

[autumnull]

Description

This pull request fixes certain markdown rendering issues by changing the rendering backend from blackfriday to goldmark.

Closes #1240

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

This looks great!! @blackle and @illfygli do you have any comments? Tagging you both because you've worked in these parts of the code :)

[blackle]

looks pretty good! I can't remember the issue I fixed, but I did add a test case for it. If all the tests are still passing, then I'm good with it.

[illfygli]

Looks good and cool, this is what I was wishing for on matrix when I did the hashtag stuff!

[autumnull]

@blackle i think ur talking about #783 and yeah it still passes! because of the fact that i made dedicated inline parsers for hashtags and mentions so they only get converted where any other inline markdown would be valid (i.e. not in code blocks).

there's also #789 which this PR doesn't fix. but i'll comment more on that issue itself if this gets merged cos i have notes.

[igalic]

how does this new markdown library deal with handles of the form: @_foo_@example.com?

glitch-soc has trouble with this: glitch-soc/mastodon#1464

[autumnull]

@igalic mentions and hashtags now have dedicated inline parsers, so any underscores appearing in hashtags or mentions cannot be interpreted by any other inline parser. they're parsed on a first-seen basis, so once the @ sign is seen then it catches everything up until the end of the mention.

In fact this applies even when the mention doesn't reference a known user, as long as it matches the mention regex. It will shield anything from markdown parsing if it looks like a mention.

e.g. the status @_foo_@example.com renders as

<p>@_foo_@example.com</p>

if the user is not known.