Provide a UI to add maintainers to a project

[carpodaster]

PR #964 introduced support for multiple maintainers, but it doesn't come with a UI. See https://github.com/rails-girls-summer-of-code/rgsoc-teams/wiki/How-to:-assign-multiple-maintainers-to-a-project

The goal is to provide a web UI for both organizers and the project submitter to invite additional users to the project as maintainers. The process could be similar to adding coaches to a team, including the requirement of confirming to be added.

[theomelo]

Is this still a thing? Would love to help!