Post-Vue-dashboard file moves

[jcushman]

This PR makes a bunch of cleanup moves now that we're moving the Vue dashboard out of a developer flag. These tend to step on file history, so I'm doing them in one big batch in the hopes of getting that out of the way:

• Move everything out of the developer-playground flag. We're keeping the flag itself for potential use later.
• Re-indent the new Vue files in frontend/ with two spaces, which is what we generally use for HTML/CSS/JS. (It would make sense at some point to adopt ESLint and Prettier, but I just did a quick effort with Pycharm for now since the files are moving anyway, in the hopes of reducing the diff size for that effort later.)
• Delete all of the now-unused javascript from the old create page.
• Move the frontend/ folder to static/frontend/. In working with Vite I found that it was hard to get both developer and production builds working with the right paths when we had Vue and CSS in different static folders.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.73%. Comparing base (f3d5fd4) to head (62019e9).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3619   +/-   ##
========================================
  Coverage    69.73%   69.73%           
========================================
  Files           54       54           
  Lines         7326     7326           
========================================
  Hits          5109     5109           
  Misses        2217     2217           

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[bensteinberg]

LGTM! I'll merge and deploy to stage. I think this will now require pretty thorough QA on stage before deploying to prod; if we need to do a bugfix before that QA happens, we might have to revert this PR and maybe the previous one, #3618, before applying the fix.