[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

merge from 3.19 branch

[entity/rset] replace scary warnings about ambiguous rdefs by XXX in the code users can't do anything about that and shouldn't see those warnings.

[web/js] Deprecate baseuri() and replace with BASE_URL (closes #3955372) Added by commit 8bf2337a6f3b and publicly available since 3.11.2 released more than 3 years ago.

[request] Ensure base_url() always has a trailing '/' (closes #3955093) Just as an example, cubicweb.ajax.js is clearly written under the assumption that BASE_URL/baseuri() - which simply contains req.base_url() - has a trailing /.