Implement abandoned bot

[iakunin]

• check the date of last push to the repo
• report when the diff with current date is more than threshold (1 year)
• report text: Repo has no activity during <period of time>
• set meta marker once reported
• don't report anymore, while the meta marker is present
• report and remove the marker if the diff is above the threshold
• report text: Repo is alived <period of time> ago

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• implement bot
• cover it by tests
• append README