Student dashboard: form dialogs, new actions and more natural UI for student groups

UI:

• Support for opening a form dialog when an action requires some information, whether it's known in advance or detected while running the action.
• Dropdown menu for the following new actions:
  • share with (team work)-
  • set main submission (team work)
  • remove submission
  • merge from another submission (WIP to be finished in a latter MR)

Model:

• Refactor Assignment.fetch by extracting a Assignment.merge_from method
• Simplify the student group / leader logic to make it more stateless (to be pursued in a follow up MR) (!176 (merged))
• Lift share_with to an assignment method
• New argument initialized for Project.ensure_fork and Assignment.ensure_submission_repo to ensure that the project should be initialized with the content of the origin repository upon creation (!176 (merged))
• GitLab.get_branch: raise ResourceNotFound if the branch is missing (minor backward incompatibility) (!176 (merged))

Tests:

• Improved test robustness with test (rich) course paths and names parametrised by test run id (!172 (merged))
• new fixture: rich_course_deployed (!174 (merged))
• new fixture utility: to_be_teared_down to mark a resource to be teared down at end of test (!172 (merged))
• New context manager: travo.util.working_directory (!172 (merged))
• Support for tests involving multiple gitlab users by mean of a context (Gitlab.logged_as) (!174 (merged))
• Default collection of users with standardized names (including travo-test-etu->student1 and blondin_al->student2) (!167 (merged))
• Multiuser tests for Assignment.merge_from, Course.collect, test_course_generate_and_release (!167 (merged))
• Fixed test_collect for courses and assignments to run in a temporary directory (!172 (merged))
• Systematically create GitLabTest instances through the gitlab fixture to guarantee uniqueness (and document why) (!174 (merged))

Misc minor fixes and doc improvements (!171 (merged))

This is a reboot of !101 (closed), after rebase on top of master.