Filter the Deprecations page by removal_milestone

Problem

The Deprecations page contains a lot of information that is difficult to parse. People are always asking how to view all of the breaking changes, or how to figure out just what's changed in a specific release.

Proposed solution

Implement filtering on the page. Start by creating filters for breaking_change: and by removal_milestone:.

Show all information by default.

Eventually we would also like to add removals to this page, and give you the ability to filter by them.

How to do it

• The source yaml files are in this directory.
• The sections are scaffolded ahead of time here.

So to make it work, you would:

1. Create a new layout with two dropdowns.
2. Show all by default, hide on dropdown triggering(new JS file for this page)
3. Ensure our styles match the GitLab UI effort