Partially addresses #47424.

These were just shorthands for project_... and they will be confusing when introducing group runners so we should not have them (#10244)

Closes #38741

Group milestones can only be referred to by name, not IID. They also do not
support cross-project references.

Rename other URL generators

admin_builds_path -> admin_jobs_path

Fix tests and more renaming

Fix more tests

Also change build_id to job_id in the controller

Remove slash commands from Markdown preview and display their outcome next to
the text field.
Introduce new "explanation" block to our slash commands DSL.
Introduce optional "parse_params" block to slash commands DSL that allows to
process a parameter before it is passed to "explanation" or "command" blocks.
Pass path for previewing Markdown as "data" attribute instead of setting
a variable on "window".

Added tests to verify the access policy to the new controller

This controller is going to contain both the project members and groups options
for the settings gear.

Generated the route and modified the routing to point to the new members setting path

Adding the necessary API for the new /snippets Restful resource
added with this commit. Added a new Grape class `Snippets`, as
well as a `PersonalSnippet` entity.

Issue: #20042
Merge-Request: !6373
Signed-off-by: Guyzmo <guyzmo+gitlab+pub@m0g.net>

Failed build:
https://gitlab.com/gitlab-org/gitlab-ce/builds/5116496

Reason why I can't see this:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16958250

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16957939

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16852207

Also incorporate feedback

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5142#note_14347729