This new attribute is optional and used when set instead of the default
format `gitlab+deploy-token-#{id}`.

Empty usernames will be saved as null in the database.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/50228.

Introduce default_git_depth in project's CI/CD settings and set it to
50. Use it if there is no GIT_DEPTH variable specified. Apply this
default only to newly created projects and keep it nil for old ones
in order to not break pipelines that rely on non-shallow clones.

default_git_depth can be updated from CI/CD Settings in the UI, must be
either nil or integer between 0 and 1000 (incl).

Inherit default_git_depth from the origin project when forking projects.

MR pipelines are run on a MR ref (refs/merge-requests/:iid/merge) and it
contains unique commit (i.e. merge commit) which doesn't exist in the
other branch/tags refs. We need to add it cause otherwise it may break
pipelines for old projects that have already enabled Pipelines for merge
results and have git depth 0.

Document new default_git_depth project CI/CD setting

Add settings for linking to external dashboard from
metrics dashboard

This new table will be used to store the external_dashboard_url which
allows users to add a link to their external dashboards (ex Grafana)
to the Metrics dashboard.

- concerns
- dashboard
- groups
- import

- Move some specs into contexts
- Let get_slugs method take a parameter and return a specific slug.
- Add rescues when using Addressable::URI.