This adds a post-processor step to the markdown parser to parse raw
html images `<img>` and resolve their `src` attributes if needed

This commit adds spinners to what I believe to be all the buttons on the
following pages:
- follow: add spinner to follow button
- issues/new: add spinner to create button
- issue: add spinners to close/reopen issue buttons
- issueComment: add spinner to delete button
- editIssueComment: add spinner to check mark button
- pulls/new: add spinner to create button
- pullNewComment: add spinner to comment button, submit and cancel
- pullResubmit: add spinners to resubmit / cancel button
- settings: add spinners to buttons
- knots: add spinners to buttons
- settings: add spinners to user settings page

Fixes a bug reported on Discord with relative links inside a
repository's subdir would resolve incorrectly since we were naively
"absoluting" the link destination.

Now, we resolve it against the current (parent) directory. For example,
if lol/x.md has a link

[foo](./some.png) => /lol/some.png (instead of just /some.png)