Consider automatic activation unblocking

[petrdvorak]

We have repeated requests for unblocking of a blocked activation. We should implement some basic stuff so that the business users get what they typically require. The mechanism should allow to specify:

• How many times the activation can be unblocked?
• Should the activation be removed when it isn't auto-unblocked? Could address #341...
• Should the counter above be related to any reasons, or only too many failed auth attempts?
• After how long should the automatic unblock happen?
• Should the interval length be linear or exponential backoff?

Examples:

• "Unblock blocked activation after 10 minutes"
• "Unblock blocked activation after 10 minutes, but at most three times"
• "Unblock blocked activation after 10 minutes, but at most three times and then remove it"
• "Unblock blocked activation at most three times, start with 10 minutes and use exponential backoff - 20 minutes for the second unblock, 40 minutes for the last one"