[migration] don't add new elements to config._cubes in remove_cube We're expanding dependencies based on the current versions of the cubes, which may well not be the same as the ones known by the db (we're in the middle of an upgrade, after all). Listing them in config._cubes prevents us from calling add_cube() later to actually add them. Closes #4002156

[web test] repair test broken by 0872ac2a1db0 after the above cset, during test instance's url is set before the creation of core entities (ie created at database initialization), hence they have now a valid cwuri.

[dataimport] Drop reference to the 'source' column (closes #4067694). CW 3.19 drops the multi-source support. This column does not exist in 3.19.

[notification] drop on commit sending of message feature of NotificationView It duplicates feature already provided by cw.hooks.notification.notify_on_commit function. Simplify the API on the way.

[datafeed parser] enhance retrieve_url to support POSTing data and custom HTTP headers Closes #3682069

[datafeed parser] fix retrieve_url to always return urllib2.urlopen compatible output as one should expect to be able to call geturl/getcode/info on the returned object whatever the url he gave. Related to #3682069

[datafeed parser] factor out retrieve_url method from DataFeedXMLParser.parse Related to #3682069

[datafeed] Allow to override use_cwuri_as_url in configuration of a datafeed source This is necessary because in some case one will want to write parser which create entities not one-to-one related to entities from the distant source (for instance see the `vcrs` cube which create entities holding statistics for code review, while not importing review entities themselves). Ideally we should provide a way to the parser to indicate the source that cwuri should not be used, but that's a start... Closes #3670210