Bug 985080 - [australis] implement updated copy and image on fx download page

The copy and layout changes only.
The new screenshots will be provided in Bug 977465.
Bug 986174 - [australis] move content from /mobile/features to /firefox/android and redirect
Merge pull request #1897 from kyoshino/bug-985080-fx-new-copy

Bug 985080 - [australis] implement updated copy and image on fx download page
Merge pull request #1921 from kyoshino/bug-986174-firefox-android

Fix Bug 986174 - [australis] move content from /mobile/features to /firefox/android and redirect