Use defined colour for a language when available

## What does this MR do?

This MR changes the colours of the different languages in the language graph.  It now uses the colour set in Linguist instead of the first six characters of the SHA256'd language name where possible.  If Linguist has no colour defined for a given language, it falls back to the old method of finding a colour.

I talked with @connorshea about creating this MR [on Twitter](https://twitter.com/connorjshea/status/784390886222286849) a few hours earlier. Here's also an older [tweet from May](https://twitter.com/nilsding/status/737018807223496708) where we discussed some possible improvements to the graph. 

## Are there points in the code the reviewer needs to double check?

Hopefully none ;)

## Why was this MR needed?

Aesthetics.

## Screenshots (if relevant)

Before:

![language_colours_before](/uploads/6b4bac784860da746d58708bdd6bba39/language_colours_before.png)

After:

![language_colours_after](/uploads/98818ebf48ffb47e6b785120e69b0b6c/language_colours_after.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

- #12455

See merge request !6748