Add UI for changing a member's role within a group

[robertknight]

Add a "Role" column to the Members tab of the group settings UI. This displays a
dropdown with the different roles that the current user can assign to the target
user within the group. If the current user cannot change the role, the column
displays the current role as plain text.

We currently prevent the user from changing their own role since this has added
complexity. It may alter the user the actions the user can take on other users
and they could lose access to the group settings page entirely if they demote
themselves to a "Member".

Example group as user robert with owner role:

Same group as user robert2 with admin role:

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TODO:

• Confirm how we should handle demoting yourself. If we decide this action should be possible, then we should show a confirmation prompt and refresh the member list afterwards, since the action will affect the other actions you can perform. If you downgrade yourself to a member, you also currently lose access to the settings page entirely. Alternatively we could decide to disable this action entirely, which will require a change on the backend to the available actions.

[acelaya]

LGTM

[acelaya]

Can you add a comment explaining why we pick only the first item in the member.roles list and ignore the rest?

IIRC, this was designed as a list to be more future-proof, allowing to add multiple roles without breaking the API contract, but for now all members will have only one role in that list.

[robertknight]

The UI design will need to change if we support more roles, and it isn't clear exactly how we'd present and enable selection of multiple roles. To keep things simple, we just assume a single role for the moment and will update this UI as necessary if we support multiple roles later.