Using gitlab url to build links for gitlab issue tracker and add a spec. Fix rubocop warnings in path.

This commit generalizes the image upload via drag and drop so it supports all files.
It also adds access control for these files.

Make the following changes to deal with new behavior in Rails 4.1.2:

* Use nested resources to avoid slashes in arguments to path helpers.

See https://github.com/gitlabhq/gitlabhq/issues/8611.

Conflicts:
	config/routes.rb