Add more input validation to dropbox.py

[makspll]

Proposed additional input validation from settings.py otherwise wrong values produce cryptic error:

File "/app/.heroku/python/lib/python3.7/site-packages/stone/backends/python_rsrc/stone_base.py", line 141, in __init__
     assert validator is not None, 'Invalid tag %r.' % tag
 AssertionError: Invalid tag True.

found and explained here: https://stackoverflow.com/questions/67872466/error-when-uploading-viewing-certain-pictures-rendition-not-found-django-sto

[jschneier]

@makspll this can more clearly be written as if write_mode not in ["add", "overwrite", "update"]. The idea is solid though. Mind rebasing and making that change?

[makspll]

@jschneier i believe that's the branch rebased and squashed now!