By caching the number of personal SSH keys we reduce the number of
queries necessary on pages such as ProjectsController#show (which can
end up querying this data multiple times).

The cache is refreshed/flushed whenever an SSH key is added, removed, or
when a user is removed.

This makes things simpler as no scheduling is involved. Further we
remove the need for running a SELECT + UPDATE just to get the key and
update it, whereas we only need an UPDATE when setting last_used_at
directly in a request.

The added service class takes care of updating Key#last_used_at without
using Sidekiq. Further it makes sure we only try to obtain a Redis lease
if we're confident that we actually need to do so, instead of always
obtaining it. We also make sure to _only_ update last_used_at instead of
also updating updated_at.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36663

`allowed_key_types` is removed and the `minimum_<type>_bits` fields are
renamed to `<tech>_key_restriction`. A special sentinel value (`-1`) signifies
that the key type is disabled.

This also feeds through to the UI - checkboxes per key type are out, inline
selection of "forbidden" and "allowed" (i.e., no restrictions) are in.

As with the previous model, unknown key types are disallowed, even if the
underlying ssh daemon happens to support them. The defaults have also been
changed from the lowest known bit size to "no restriction". So if someone
does happen to have a 768-bit RSA key, it will continue to work on upgrade, at
least until the administrator restricts them.

This is an amalgamation of:

* Cory Hinshaw: Initial implementation !5552
* Rémy Coutable: Updates !9350
* Nick Thomas: Resolve conflicts and add ED25519 support !13712

Don't include EmailHelpers manually, pick with rspec

See merge request !13257

`:mailer` is needed to pick it easily, while
`type: :mailer` is needed for picking it automatically for
tests located in spec/mailers/*_spec.rb

It's a bit complicated in spec/services/notification_service_spec.rb
but we'll leave it alone for now.