Merge branch 'cs-fix-firefox-height' into 'master'

Fix line numbers not matching up with code in Firefox.

Firefox has had broken line number alignment for a while now, if this is a bug with Firefox they don't seem to want to fix it since its been there from 46 through 49. This fixes it.

Before:

![Screen_Shot_2016-09-22_at_3.08.44_PM](/uploads/8bb65f40e44f738c0c375b0c8ef7dea6/Screen_Shot_2016-09-22_at_3.08.44_PM.png)

After:

![Screen_Shot_2016-09-22_at_3.09.10_PM](/uploads/9e537be56886569d8a82390f2b97c42b/Screen_Shot_2016-09-22_at_3.09.10_PM.png)

Fixes #20202.

Safari/Chrome work the same as before, Snippets seems to work fine still as well.

Page tested: http://localhost:3000/gitlab-org/gitlab-ce/blob/master/app/models/project.rb

cc: @annabeldunstone

See merge request !6485

app/assets/stylesheets/framework/highlight.scss

@@ -11,7 +11,7 @@
     border-radius: 0;
     font-family: $monospace_font;
     font-size: $code_font_size;
-    line-height: $code_line_height !important;
+    line-height: 19px;
     margin: 0;
     overflow: auto;
     overflow-y: hidden;
@@ -47,7 +47,7 @@
       font-family: $monospace_font;
       display: block;
       font-size: $code_font_size !important;
-      line-height: $code_line_height !important;
+      line-height: 19px;
       white-space: nowrap;
 
       i {