Revisit blockchain connect/approval process

[kevinbader]

New features:

• cancel join request
• take back approval (as long as request is still pending)
• request to revoke another key's permissions (e.g., revoke admin permissions), which should trigger another voting
• a voting process should have a timeout
• requests that have been rejected or timed out should no longer be visible

[stale[bot]]

This issue has been automatically marked as stale because it has not had activity for 30 days. It will be closed if no further activity occurs. Thank you for your contributions.