Merge branch '19003-file-view-subnav' into 'master'

Resolve "Sub nav isn't showing on file view"

## What does this MR do?
Adds subnav to `Repository` > `File` view

## What are the relevant issue numbers?
Closes #19003 
Part of #18844 

## Screenshots (if relevant)
![Screen_Shot_2016-06-23_at_5.33.05_PM](/uploads/aa6993b2376dbe454af87d852aa74f5e/Screen_Shot_2016-06-23_at_5.33.05_PM.png)

cc @dzaporozhets 

See merge request !4890