Expose time since starring on project/:id/starrers API endpoint; exclude private profiles here as well

Fix API endpoint for starred projects of a user; add info about starred projects on profile to documentation

Save instance administration project id in DB

See merge request gitlab-org/gitlab-ce!31111

- This will make it easy to identify the project even if admins change
the name of the project or move it.

Upgrade Gitaly to v1.57.0

See merge request gitlab-org/gitlab-ce!31568

Add Documentation for Feature Flag Percent Rollout

Closes gitlab-ee#8240

See merge request gitlab-org/gitlab-ce!29770

Add updated screen shots

Add Jenkins quick reference page

See merge request gitlab-org/gitlab-ce!31518

Support dashboard params for metrics dashboard endpoint

See merge request gitlab-org/gitlab-ce!31063

https://gitlab.com/gitlab-org/gitlab-ce/issues/62971

Adds support to EnvironmentsController#metrics_dashboard
for the following params: group, title, y_label
These params are used to uniquely identify a panel on
the metrics dashboard.

Metrics are stored in several places, so this adds
utilities to find a specific panel from the database
or filesystem depending on the metric specified.

Also moves some shared utilities into separate classes,
notably default values and errors.