Feature request: Cancel team join request

When clicking on a team name like `@europe`, it's easy to immediately click on the next visible button, which immediately sends a request to join that team.

It would be helpful to have an option to cancel such a request by ourselves, instead of the team admin having to disapprove that request.

Android user here, if that's helpful.

Start by tracing the Android flow from tapping a team name through the visible join-request action. Identify where a pending request is represented and how the current request reaches team administrators; done means a user can cancel their own pending request without administrator intervention, with the behavior covered by the relevant tests.

Written by the indexing model from the issue text.