Don't attempt Rate-Limiting if app.redis is not set
Default Rate-Limit set to 300 requests/15 minutes
'utils.request_method' helper function

'utils.request_method' returns flask's request.method, but taking our
support for PATCH alternative (POST with the 'X-HTTP-Method-Override'
header) into consideration. This helper function allowed for some nice
refactoring in the code.
Rebase conflict solved