Merge branch 'convert-switch-icon-to-font' into 'master'

Convert switch icon into icon font

## What does this MR do?
Converts the switch icon into the exchange icon from font awesome

## Are there points in the code the reviewer needs to double check?
Shouldn't be any

## Why was this MR needed?

*  Decreases an external resource for the repository compare page
*  Icon font provides a sharper image for displays with higher pixel resolutions

## What are the relevant issue numbers?
Closes #20508 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-01_at_3.06.06_PM](/uploads/6ad46cc6dff98ec534e1f5a0fe556fa7/Screen_Shot_2016-08-01_at_3.06.06_PM.png)

After:
![Screen_Shot_2016-08-01_at_3.06.10_PM](/uploads/3cb11c40cbba398f9986a4cc6b3a7e4c/Screen_Shot_2016-08-01_at_3.06.10_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5603

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.11.0 (unreleased)
   - Fix the title of the toggle dropdown button. !5515 (herminiotorres)
   - Improve diff performance by eliminating redundant checks for text blobs
+  - Convert switch icon into icon font (ClemMakesApps)
   - Remove magic comments (`# encoding: UTF-8`) from Ruby files. !5456 (winniehell)
   - Add support for relative links starting with ./ or / to RelativeLinkFilter (winniehell)
   - Fix CI status icon link underline (ClemMakesApps)

app/assets/stylesheets/pages/commits.scss

 .commits-compare-switch {
   @include btn-default;
   @include btn-white;
-  background: image-url("switch_icon.png") no-repeat center center;
-  text-indent: -9999px;
   float: left;
   margin-right: 9px;
 }

app/views/projects/compare/_form.html.haml

 = form_tag namespace_project_compare_index_path(@project.namespace, @project), method: :post, class: 'form-inline js-requires-input' do
   .clearfix
     - if params[:to] && params[:from]
-      = link_to 'switch', {from: params[:to], to: params[:from]}, {class: 'commits-compare-switch has-tooltip', title: 'Switch base of comparison'}
+      = link_to icon('exchange'), {from: params[:to], to: params[:from]}, {class: 'commits-compare-switch has-tooltip', title: 'Switch base of comparison'}
     .form-group.dropdown.compare-form-group.js-compare-from-dropdown
       .input-group.inline-input-group
         %span.input-group-addon from