Exclude GET /publish /share URLs from 500's.

[cycomachead]

These routes shouldn't do anything, but google reports them as 500's and tries to index them, which feels wrong.
I think Berkeley's accessibility script also tries to crawl these pages and reports errors.

[cycomachead]

Sentry also shows a lot of /mark_as_remix and /flag routes.

Maybe we need some get of

GET = cannot_use_get(route) handler that we can call in a few places. It should render HTML or JSON, but return a 406.