no leading slash at project reference

This adds a limitation that we will try to create pipeline
for at most 4 first changes (branches and tags).

This does not affect processing of Pipelines for Merge Requests,
as each updated MR will have associated pipeline created.

CI ref pattern matching now uses RE2 instead of Ruby.
See
https://about.gitlab.com/2019/04/01/security-release-gitlab-11-dot-9-dot-4-released/ for more information.

This adds documentation for:

- `CI_BUILDS_DIR`,
- `CI_CONCURRENT_ID`,
- `CI_CONCURRENT_PROJECT_ID`,
- `GIT_CLONE_PATH`,
- `GIT_CLEAN_FLAGS`

This brings support for untrusted regexp for 'only:refs:' when
enabled via feature flag: alllow_unsafe_ruby_regexp.

This is by default disabled, and should not be used in production

At-symbols are used to match a ref to a specific repository path.
Currently this is done by string splitting the ref pattern on the
at-symbol. In order to pattern match a literal `@` you must use
ruby's unicode character matching `\u{0040}`.

Using we and you isn't very pleasant to read nor is it professional. So
lets remove these invocations to have a more pleasant read.
Signed-off-by: NOlliver Schinagl <oliver@schinagl.nl>

This makes ref validation to use always `UntrustedRegexp`.
This also splits the existing RubySyntax into separate
class.

The 'master' branch name is confusing as it indicates some special branch name,
where it is actually just a 'ref'. 'branches' _is_ a special name as it
indicates 'all' branches.

Lets clarify the example a little to indicate this.
Signed-off-by: NOlliver Schinagl <oliver@schinagl.nl>

Also refactors structure of the page
to make the first section more useful.

This change introduces a support for nesting the includes,
allowing to evaluate them in context of the target,
by properly respecting the relative inclusions and user permissions
of another projects, or templates.

Restructures existing page and adds improvements.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/56172