Fix wiki linking behavior for markdown wiki pages
Related to #18019
- As per the documentation in !4372
## TODO
- [ ] !4432 Have wiki linking behave as per the documentation
- [x] Move `WikiLinkFilter` specs to the pipeline level
- [x] Verify current behavior on wiki `show` page
- [x] Fix current behavior on wiki `show` page
- [x] Verify current behaviour on wiki preview
- [x] Fix current behaviour on wiki preview
- [x] Rewrite all links and get preview links working
- [x] Make sure all links are on-par with the wiki `show` page
- [x] TDD `WikiLinkFilter` and get it working
- [x] Hook `WikiLinkFilter` up
- [x] Fix tests
- [x] Fix `markdown_spec`
- [x] Fix `wiki` spinach feature
- [x] Wait for [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/4f50dd2/builds) to pass
- [x] Make sure all wiki-related pages are working as expected (history, all pages, etc.)
- [x] Test in different ruby versions
- [x] GitLab instances hosted on a relative URL
- [x] Non-markdown rendering formats?
- [x] RDoc
- [x] ASCIIDoc
- [x] Create issues to fix things for RDoc and ASCIIDoc
- [x] Gauge performance impact
- [x] Refactor
- [x] Re-organize commits
- [x] Make sure [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/f860e9a8dcabe7d5f160c32fc549807c98baa4a1/builds) passes
- [x] Respond to @rymai's comments
- [x] `class WikiLinkFilter < HTML::Pipeline::Filter`
- [x] blank line after guard clause
- [x] keyword arguments for `wiki` and `slug`
- [x] invert the condition
- [x] inline `user` in spec
- [x] Make sure spec names are not out of date
- [x] Comment for each rewrite rule
- [x] Add CHANGELOG entry
- [x] Reorganize commits
- [x] Make sure [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/19b91e749a6320d12fb299d33f1f6440777e0e26/builds) passes
- [ ] Wait for merge
See merge request !4432
- Need to pass in a `page_slug` to the filter, so it can rewrite based
on the current page (all links are rewritten to the level of the app root).
- The earlier `markdown_preview` endpoint was at the level of the wiki.
We need to know the current page (for rewriting, as above), so this
commit moves the endpoint to the level of a wiki page.
- Fix all tests
Notifications dropdown on project page now has descriptions
## What does this MR do?
Adds descriptions to the notification settings dropdown on project pages. Also changed the order of them to make more sense.
## What are the relevant issue numbers?
Part of #12758
## Screenshots (if relevant)

See merge request !4374
Implement compact side nav
## What does this MR do?
Implements new compact nav design
Note- new navbar icons are in a separate MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4268)
## What are the relevant issue numbers?
Closes#18013
## Screenshots (if relevant)


cc @dzaporozhets
See merge request !4447
Use Knapsack to evenly distribute specs
Try to distribute all tests evenly accross all nodes to reduce the testing time of GitLab CE.
This uses high amount of auto-scaled machines.
See merge request !4240
POC: Updated contrib calendar
In an effort to cut down the JS file size - i've removed the heatmap calendar used for the contributing calendar on users profiles. We already have d3 on the page so why not use it instead of using a library which uses it?

cc. @jschatz1
See merge request !3944
Fix grammar and wording in text on various pages.
## What does this MR do?
Fixes various grammatical and/or wording issues I found while copyediting on the plane :)
## Why was this MR needed?
Incorrect grammar and weird/confusing wording makes the product look less polished and professional.
See merge request !4237