add checkbox to toggle show/hide hidden files
persist show hidden setting
fix settings menu layout
test ApiController::showHiddenFiles
don't show hidden files by default
Store config in Backbone model and inject it into FileList
Filter files only temporarily when rending the file list
Fix file rename validation
This is required by the pdf viewer, since the files app on master uses the AppFramework it had applied the more strict defaults which made it not work on master.