Conflicts:
- `README.md`:
Minor upstream change, our README is completely different.
Kept ours.
- `lib/tasks/assets.rake`:
glitch-soc has extra code to deal with its theming system,
upstream changed a line that exists in glitch-soc.
Applied upstream changes.
- 500.html generated with admin-set default locale if set
- Error page `<title>` includes Mastodon site title
- 500 title changed to "This page is not
correct" (ref: <https://www.youtube.com/watch?v=2VCAP_seh1A>)
- 500 content appended with "on our end" to make clear it's
not user's fault
