Building on support for setting feature flags by project, this adds
support for setting them by GitLab group path.

This is different from setting them by Flipper feature_groups, which
are for batch updating pre-registered collections.

For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.

- Breaks up into more sections.
- Also minor fixes to pages within sections.

This reverts merge request !25173

Extend the list of supported protocols to include FTP.

Issues
https://gitlab.com/gitlab-org/gitlab-ce/issues/47149
https://gitlab.com/gitlab-org/gitlab-ce/issues/56014

Cached markdown version is composed both from global and local
markdown version. This allows admins to bump version locally when
needed (e.g. when external URL is changed).

In order to have an accurate date about the last activity of a User
we need to update the last_activity_on field when the User is visiting
some basic pages of GitLab like pages related to Dashboards, Projects,
Issues and Merge Requests

This includes a set of APIs to manipulate container registry.
This includes also an ability to delete tags based on requested
criteria, like keep-last-n, matching-name, older-than.