Fill missing authorized projects rows

Closes #26194

See merge request !8476

Remove more orphans when removing stray namespaces

Closes #25146

See merge request !7841

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

This ensures that the project_authorizations rows exist for all users
for which this data has not yet been populated.

Fixes #26194

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25146

Remove the project_authorizations.id column

See merge request !8479

Record and show last used date of SSH Keys

See merge request !8113

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.