Fix docs on artifacts API

Small fixes for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2893

- Backticks in parameters
- Remove headings
- Fix curl URL link



See merge request !2902

- Backticks in parameters
- Remove headings
- Fix curl URL link

[ci skip]

Move a few controller specs to their correct locations



See merge request !2889

Add a `Blob` model that wraps `Gitlab::Git::Blob`

This allows us to take advantage of Rails' `to_partial_path` to render
the correct partial based on the Blob type, rather than cluttering the
view with conditionals.

It also allows (and will allow in the future) better encapsulation for
Blob-related logic which makes sense for our Rails app but might not
make as much sense for the core `gitlab_git` library, such as detecting
if the blob is an SVG.

See merge request !2887

Improve readability of 'rake cache:clear' code



See merge request !2892

Introduce API for serving the artifacts archive

Add API to download build artifacts archive in context of GitLab API

/cc @DouweM @grzesiek 

See merge request !2893

Add documentation about build emails service

Closes #12497

See merge request !2871