Team member promotion demotion

[begedin]

• Asana task: Team management step 2.2 - Implement promotion of members to admin - Ember
• This branches off of #18 so we need to merge that one before the changeset here starts making any sense.

  Description

The tests and features are done

A user can be assigned the admin or member role. The UI makes it look like a toggle button between the two. Once and if we have more roles, a dropdown might make more sense.

If the membership in question is just an invite, or it belongs to the team owner, then the role cannot be modified and the buttons are not rendered.

If the current user is not an admin or an owner, the API will return an error.

Ideally, we would hide the buttons in that case, but I'm working on that approach in #20, where I did a good amount of refactoring of how the current user is handled.

[begedin]

Marked this as awaiting review, but integration tests PR should be merged first, before this can be effectively reviewed.

[venkatd]

@begedin if you get the integration tests merged I can look at the code changes here