Add a fake User-Agent to tasks fetching remote resources (bug 1018279)

https://bugzilla.mozilla.org/show_bug.cgi?id=1018279

Sadly necessary because some hosts blacklist the default UA used by python-requests :( I used the UA that the validator was already using when fetching content itself [0], and at the same time didn't want to import it because it's not just a validator issue : we also need that when fetching icons for instance.

[0] https://github.com/mozilla/app-validator/blob/master/appvalidator/testcases/webappbase.py#L17
is this what the manifest retry code (the daily manifest fetcher) checks too?