REST: Don't iterate through models at class level

REST: Don't iterate through models at class level

This causes two issues. Firstly, on fresh installs you see the following
error message:

  "Table 'patchwork.patchwork_state' doesn't exist"

Secondly, any new states created when the process is running will not be
reflected in the API until the server process is restarted.

Resolve this issue by moving the step into a method, thus ensuring it's
continuously refreshed. It doesn't seem possible to add tests to prevent
this regressing but some similarly useful tests are included to at least
validate the behavior of that field.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Tested-By: Denis Laxalde <denis@laxalde.org>
Fixes: a2993505 ("REST: Make 'Patch.state' editable")
Closes-bug: #67
Closes-bug: #80