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

Addresses: Issue #13810

1. Adds a last_used_at attribute to the Key table/model
2. Update a key's last_used_at whenever it gets used
3. Display how long ago an ssh key was last used

This column used to be a 32 bits integer, allowing for only a maximum of
2 147 483 647 rows. Given enough users one can hit this limit pretty
quickly, as was the case for GitLab.com.

Changing this type to bigint (= 64 bits) would give us more space, but
we'd eventually hit the same limit given enough users and projects. A
much more sustainable solution is to simply drop the "id" column.

There were only 2 lines of code depending on this column being present,
and neither truly required it to be present. Instead the code now uses
the "project_id" column combined with the "user_id" column. This means
that instead of something like this:

    DELETE FROM project_authorizations
    WHERE user_id = X
    AND id = Y;

We now run the following when removing rows:

    DELETE FROM project_authorizations
    WHERE user_id = X
    AND project_id = Y;

Since both user_id and project_id are indexed this should not slow down
the DELETE query.

This commit also removes the "dependent: destroy" clause from the
"project_authorizations" relation in the User and Project models.
Keeping this prevents Rails from being able to remove data as it relies
on an "id" column being present. Since the "project_authorizations"
table has proper foreign keys set up (with cascading removals) we don't
need to depend on any Rails logic.

Revert MattermostNotificationService and SlackNotificationService to MattermostService and SlackService

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

We cant have project with name 'project' or 'tree' anymore.
This merge request containts a migration that will find and rename all
projects using reserved names by adding N digit to the end of the name.
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

This adds counters for build artifacts and LFS objects, and moves
the preexisting repository_size and commit_count from the projects
table into a new project_statistics table.

The counters are displayed in the administration area for projects
and groups, and also available through the API for admins (on */all)
and normal users (on */owned)

The statistics are updated through ProjectCacheWorker, which can now
do more granular updates with the new :statistics argument.

This adds a migration to remove unused services, where the properties
are empty. As the properties are empty, those do not contain any
settings or other information.

Fixes #25727

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

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

Currently namespace name and path have uniq validaiton which does not
allow us to use same group name/path inside different groups. This
commit changes validation in next way:

* Allow same namespace name with different parent_id
* Allow same namespace path. Uniq validation should be handled by routes table
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

* add parent_id field to namespaces table to store relation with nested groups
* create routes table to keep information about full path of every group and project
* project/group lookup by full path from routes table
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

This new global setting will allow admins to specify if HTML emails should be sent or not,
this is basically useful when system administrators want to save some disk space by avoiding
emails in HTML format and using only the Plain Text version.

Closes #23150

This prevents leakage of project names on an endpoint which is
unauthenticated and thus open to the world.

The database should not have any soft-deleted groups. Due to a race
condition (soft-delete completes after the hard-delete), soft-deleted
groups were (incorrectly) left in the database, causing issues while
trying to create a new group with the same name.

add pipeline id to merge request metrics table. Also, updated the pipeline worker to populate this field.