Fixes #587. Notifiy when node is down for too long.

* Update notification NodeNotAvailable.
* Update offline time to 5 days.
* Handle flapping states.
Add support to configure notifications expiration.

You can define the period while a delivered notification
remains valid. After that, notification becomes expired
and new emails could be sent.
Group notifications by set_state.
Migrate data: sliver_data XOR sliver_overlay (#200)
Remove overlay fields of sliver (#200).
Merge branch 'master' into feature200

Apply feature236 changes into feature200.
Fix typo in warning about overlay settings removed.
Grant {Node|Server}Api base_uri always ends with a slash '/'.
Cosmetics. Fix layout and improve build change view.
Improve gateways dropped backwards compatibility. Fixes #236.

Keep a snapshot of current gateways to provide readonly api views
to keep backwards compatibility with old CNS.
Remove UCI base_path (#245 note-57).

confine.server.base_path not needed anymore during FW generation.
Fix tinc default gateway on clean installation.
Refactor: call commands following docs recommendation.
Cosmetic changes to offline nodes notification (#587).
Revert "Refactor: call commands following docs recommendation."

This reverts commit ec7486dd1e8776dc965268daf4c0fa7af94ce091.
Mark default connect to as required to nodes and hosts.
Hide default_connect_to on node@registry. Fixes #157.
Merge branch 'master' into bug587
Merge branch 'bug587'
Revert "Mark default connect to as required to nodes and hosts."

This partially reverts commit 0614da2132ac9c543e7702b4c7bd13c5a4e03796.