Revisit blockchain connect/approval process

[Stezido]

Discussed in #877

^{Originally posted by kevinbader April 3, 2019}
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

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