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.
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.
Fix tinc address admin search.
Implements TincAdress permissions. refs #236.

* Registered user: CRUD addresses on their hosts.
* Slice admin: same as registered.
* Node and group admin: members + CRUD on their nodes.
* Superusers: CRUD addresses on hosts, nodes and servers.
Validate tinc.default_connect_to on nodes and hosts.
Handle None on sizeof_fmt.