Create Differences page

Commit 0e25b16 contains the new form that displays the overrides of a Feature (diffs).

Also had to change {name} to {featurename} in the routing.yml file to avoid conflicts with the config_update module that also uses {name} in it's routing. Apparently the arguments from different local tasks are shared and need to be unique across modules. Not sure if this is a core bug or "by design". But when passing a url like:

admin/config/development/configuration/features/diff/my_feature

it was trying to use "my_feature" as the {name} argument in the /admin/config/development/configuration/report/{report_type}/{name} route within config_update module.

Commit 2f0d791 is more work on the Differences page. Uses the tableselect form api to give a list of config and added the form action to revert (import) the differences.