Use automated whitespace trimming in Jinja

Indented the navbar list items and removed manual whitespace controls in base.html and macros.html
Refactor the render_field macro
Remove extra newline in status_color macro
Remove extra whitespace in request listing table rows
Change stupid GET-forms with normal links

I must've been really tired when I wrote this originally.
Add httmock to requirements.txt
Add common mocks to tests.util

tests.util.response is a wrapper around HTTMock.response to work around an issue where the _content attribute is a string when Requests is expecting a bytes object.
Slim down the ZKillboard responses

Omitting attackers and items lists.
Handle CREST errors better

CREST actually returns appropriate response codes for errors.
Have ZKillmail check HTTP response code

In the hope that someday ZKillboard will return appropriate status codes.
Use HTTMock in testing the request views
Use HTTMock for testing killmails