Confirm more 'destructive' actions

[huntertur]

• Add confirmations for the following actions:
  • unfollowing a user
  • unfavoriting a submission
  • ignoring a user
• Replace one-off confirmation logic for unfriending a user with data-confirm
• Replace XMLHttpRequest with fetch in client-side submission favoriting logic

I was not able to cleanly use data-confirm for unfavoriting a submission, since the next submit event handler, which does the web request to favorite or unfavorite the submission, will then immediately fire regardless of the user's choice for the confirmation dialog. Perhaps the e.preventDefault(); in the data-confirm handler could be changed to ignore all following event handlers instead of just the default event handler, but I didn't pursue this since I don't know if this would break another part of Weasyl.

Closes #1462.