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

Bắt đầu bằng việc xem xét quy trình thêm thành viên hiện tại và các issue liên quan #194, #134 và #123. Lập bản đồ hành vi được yêu cầu đối với admins, founders hoặc maintainers và những người dùng khác, sau đó xác định tiêu chí hoàn thành dựa trên các điều khiển theo từng vai trò, việc tự tham gia và các vai trò dự án do founders gán.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.