WIP: user can upload an OPML file to import his subscriptions data from another service.

A background job is enqueued to process the uploaded file, if no errors happen.

Ensure that if an error happens when uploading the file (e.g. the file is a zip with no xml or opml files inside) the user is redirected to the feeds view anyway.
Changed the templates for the import process views, so that colors and icons are different depending on the status of the import process for the logged user (not started, running, success or error).
WIP: while the import process is running, a javascript timed function is periodically fetching the state of the import process and updating the view accordingly.