Support OPML upload

[mpgirro]

OPML (Outline Processor Markup Language) files are used to share Podcast subscriptions between Podcatchers. Thus they contain feeds of podcasts. We can realise bulk-propose operations if we enable the upload of OPML files. The Engine need to validate that they these files are truely OPML, extract the feeds, and perform a propose operation for each of them.

This affects the following subprojects:

• Engine: Must implement the whole processing logic
• API: Must provide an endpoint to send the OPML files too
• Web (Beta): Must enable a Form (on the +feed page) where we can select the file to upload.