Make request.useragent column in database nullable

[FastLizard4]

Currently, the request.useragent column in the database is defined as TEXT NOT NULL, which means that any requests we receive where we don't receive a User-Agent HTTP header will cause an exception and fail. Currently we only see this with requests that appear to be fuzzed for automated scanning or similar, but it's still not an ideal behavior.

To fix, we should make the request.useragent column nullable.

[FastLizard4]

Note that there's currently a handful of open PRs that make database changes (#852, #854, and #856) so it's probably best for those to get merged in before working on this (or, alternatively, base any change for this issue on the branch for #856).

[stwalkerster]

This is a good first issue This task is suitable for developers new to the project candidate, as the change required should be very similar to #713

I won't tag it as such yet due to the other stuff pending.