`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

In favor of the new `repository_storages`

When sign-in is disabled:
 - skip password expiration checks
 - prevent password reset requests
 - don’t show Password tab in User Settings
 - don’t allow login with username/password for Git over HTTP requests
 - render 404 on requests to Profiles::PasswordsController

Removed all the unnecessary include of `WaitForAjax` and `ApiHelpers` in the specs.
Removed unnecessary usage of `api:true`

Use strings for the ApplicationSetting properties:
 - restricted_visibility_levels
 - default_project_visibility
 - default_snippet_visibility
 - default_group_visibility

This MR enables rendering of PlantUML diagrams in Asciidoc documents. To add a
PlantUML diagram all we need is to include a plantuml block like:

```
[plantuml, id="myDiagram", width="100px", height="100px"]
--
bob -> alice : ping
alice -> bob : pong
--
```

The plantuml block is substituted by an HTML img element with *src* pointing to
an external PlantUML server.

This MR also add a PlantUML integration section to the Administrator -> Settings
page to configure the PlantUML rendering service and to enable/disable it.

Closes: #17603

Restore changes for api spec files

Fix error in rspec Users

Delete extra space Repositories-spec

This will allow the Koding app to enable the integration itself once is has authorized an admin user using the application secrets.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>