Fix issue with delegation of patch via REST API

Fix issue with delegation of patch via REST API

There have been reports of people being unable to delegate patches to
themselves, despite being a maintainer or the project to which the patch
is associated.

The issue is a result of how we do a check for whether the user is a
maintainer of the patch's project [1]. This check is checking if a given
'User.id' is in the list of items referenced by
'Project.maintainer_project'. However, 'Project.maintainer_project' is a
backref to 'UserProfile.maintainer_projects'. This means we're comparing
'User.id' and 'UserProfile.id'. Boo.

This wasn't seen in testing since we've had a post-save callback [2] for some
time that ensures we always create a 'UserProfile' object whenever we create a
'User' object. This also means we won't have an issue on deployments initially
deployed after that post-save callback was added, a 'User' with id=N will
always have a corresponding 'UserProfile' with id=N. However, that's not true
for older deployments such as the ozlabs.org one.

[1] https://github.com/getpatchwork/patchwork/blob/89c924f9bc/patchwork/api/patch.py#L108-L111
[2] https://github.com/getpatchwork/patchwork/blob/89c924f9bc/patchwork/models.py#L204-L210

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #313
Reported-by: Bjorn Helgaas <helgaas@kernel.org>
(cherry picked from commit ab35df8c33a178b3b2349c1e4727393b94f5e916)