[tableview] drop useless code: cellvid systematically given by RsetTableView, according to its .[non]finalview attribute

[tableviews] use ``select_or_none`` instead of ``try: select() except:`` select_or_none is here to avoid multiplying try: except: context in the code.

[bw compat] json_request setter is necessary for some cubes which has json_request (eg preview)

gmap: set center and zoomlevel automatically (closes #2144255) Instead of hardcoding the default zoomlevel and setting the center to the average of all markers, let gmap compute them so all markers fit in the map.

[views] calendar id in CalendarView must be unique (closes #2143864)

[debian] cubicweb works with python 2.7 (which become the standard on some distros). Closes #2142050

[multi-sources] call doexec rather than sqlexec since we do not want to call process_result (as there aren't any and this breaks with lgdb default)

[sources] hide some attributes from source edition form. Closes #2144160

[https] Allow configuration of anonymous access on https (closes #1971992) For now any anonymous access in denied on secured connection (https-url). This patch introduce a configuration option to control this behaviour. New instance will allow it. Old instance should stay as is during migration.

[bfss] convert root BFSS directory to str in constructor (closes #2137793) otherwise you can get crashed with AssertionError: Binary objects must use raw strings, not <type 'unicode'>