Centralize webapi tests' mimetypes and URL lookups.
The work to split up the webapi test suite had an issue where unit test

classes were being executed multiple times in a test suite run. This

happened because test*.py files were importing classes from other

test*.py files, and nose was finding them. These needed to be imported

in order to get the mimetypes and URLs from those test classes.
This change solves this problem by introducing webapi/tests/mimetypes.py

and webapi/tests/urls.py. mimetypes.py contains the full list of all

expected mimetypes found throughout the test suite. urls.py contains

functions for fetching all the URLs for all resources.
As an aid to urls.py, WebAPIResource now includes functions for

computing the URLs. The ones in urls.py just wrap them to make the

lookups more convenient for the purpose of tests.

Unit tests pass.