User images and videos will get proxied through
the Camo server in order to keep malicious
sites from collecting the IP address of users.

Adds GFM Pipline filters to insert a placeholder in the generated
HTML from GFM based on the presence of a metrics dashboard link.

The front end should look for the class 'js-render-metrics' to
determine if it should replace the element with metrics charts.
The data element 'data-dashboard-url' should be the endpoint
the front end should hit in order to obtain a dashboard layout
in order to appropriately render the charts.

If we put the emoji filter before the reference filters, each emoji will
have a wrapper element that prevents the reference filter from detecting
the presence of the emoji.

As the emoji filter now runs after the reference filters, references
must contain a literal emoji, not the GitLab Flavored Markdown
versions (:100`, for example).

A weird side-effect is that if you have a label with the 100 emoji, and
a label named 💯, then trying to reference the latter will work (link
to the correct label), but will render with the 100 emoji. I'm
comfortable with that edge case, I think.

The spec needed to be updated because in some cases the resulting
Markdown is slightly different, though equally valid.

All the ids and classes were stripped.  Add them back in
and make ids unique

[master] XSS in markdown following unrecognized HTML element

Closes #2732

See merge request gitlab/gitlabhq!2599

Enables frozen for the following:

* lib/*.rb
* lib/banzai/**/*.rb
* lib/bitbucket/**/*.rb
* lib/constraints/**/*.rb
* lib/container_registry/**/*.rb
* lib/declarative_policy/**/*.rb

Partially addresses #47424.

A working implementation of a project reference filter which links project references to project profile.

Remove class from SanitizationFilter whitelist

See merge request !2079

Allow rendering of PlantUML diagrams in Markdown documents using fenced blocks:

    ```plantuml
    Bob -> Sara : Hello
    Sara -> Bob : Go away
    ```

Closes: #4048

Also, always add a link to download videos since video playback is
tricky. Also, it solves the issue with email client not supporting
videos.

Signed-off-by: Rémy Coutable <remy@rymai.me>

* Registered video MIME types
* Currently supporting browser-supported formats with extensions that match the mime type

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Resolves #14411.

See merge request !3464