Update PreferencesHelper specs

Instead of rendering this value server-side, we use Javascript and Gon
to apply the user's color scheme (or the default) to any syntax
highlighted code blocks.

This will make it easier to cache these blocks in the future because
they're no longer state-dependent.

Refactored HTML parser to avoid duplication of newline parsing.

Closes #2235

HTML span elements only apply to a single line, and in the case of multi-line
comments, the highlighting was dropped as a result. Ensure that each line has
the proper styling to fix this.

Closes #1577

Fix error 500 when no HEAD is available

Steps to reproduce:

1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Error:

```
NoMethodError - undefined method `sha' for nil:NilClass:
  app/helpers/projects_helper.rb:281:in `readme_cache_key'
  app/views/projects/_readme.html.haml:10:in `_app_views_projects__readme_html_haml___2036282917939462960_70154565285700'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:339:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:143:in `render'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiling_methods.rb:108:in `block in profile_method'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:306:in `render_partial'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:279:in `block in render'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:278:in `render'
  actionview (4.1.11) lib/action_view/renderer/renderer.rb:47:in `render_partial'
  actionview (4.1.11) lib/action_view/helpers/rendering_helper.rb:35:in `render'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `block in render_with_haml'
  haml (4.0.5) lib/haml/helpers.rb:89:in `non_haml'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `render_with_haml'
  app/views/projects/show.html.haml:47:in `_app_views_projects_show_html_haml__1458303859692972698_70154948164180'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
```

Closes https://github.com/gitlabhq/gitlabhq/issues/9484, but there may be some other issue there.

See merge request !1041

Steps to reproduce:
1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Closes https://github.com/gitlabhq/gitlabhq/issues/9484

Forks should not have more permissive visibility levels than the original

https://dev.gitlab.org/gitlab/gitlabhq/issues/2286

See merge request !936