Feature request: Improve "add-member" functionality based on user and project role

Current add-member to the project functionality is allowing any users to add any other user to the project.

In order to have easy to use interface for new users, I suggest splitting this functionality based on user rights and role on project. Could be added also to "improve user roles rights" topic.

Admin user:
- add-member is working as current implementation (allow to add user by adding username i.e. typeahead - see #194)
- have "+Add" button under list of project members

Project founder/maintainer:
- user who started project (founder), or
- user to whom founder transferred "maintainer" role
- should have add-member as_is (like in current implementation), see #134
- have "+Add" button under list of project members

All other users:
- currently user has to click "+Add" button, then write his username, and submit (problem: too many steps)
- should be improved, this user should be able to add himself/herself to the project with one click (i.e. autofill current user to add myself to a project, see #194)
- should see button "+Participate" meaning that user wants to be added to this project (see #123)
- user role on a project should be filled by project founder/owner

Cheers,
Miroslav

まず、現在のメンバー追加フローと、関連する issue #194、#134、#123 を確認します。管理者、創設者またはメンテナー、その他のユーザーについて求められている動作を整理し、ロールごとの操作、本人の参加、創設者が割り当てるプロジェクトロールを基準に完了条件を定義します。

索引モデルが issue の本文から書いたものです。