no user message when maintainers form is saved

- Added message
- Added assertions in project.test to make sure they show up as expected
(also tested manually, they look fine)

My only question is about making project.test assertions more specific now -- once you save the form to add a new maintainer, the message itself contains a link to the user. It'd be nice if the assertion that the new maintainer is in the table could ensure that the user link we're looking for is actually in the table, not just anywhere on the page.

I think this is what you are looking for.

Finally got around to reviewing, testing, committing, pushing, and deploying this. ;)