Introduce ability for crew members to revoke their own membership

[qurbat]

Currently, profile owners can revoke crew membership for themselves and others, whereas non-profile owners cannot. The use case requirement for crew members to be able to remove themselves is clear, but what are the potential consequences of this? What edge cases need to be thought of?

This issue is meant to track questions and concerns around this feature.

[jace]

In a project, everyone can remove themselves without side effects as the power to add members comes from being a profile admin.

In a profile, the last owner cannot leave. Everyone else can.