[management ui] add views to manage sources similarly to users

[manage view] cleanup manage view and user menu * drop code that shouldn't be there (e.g. folders handling) * add a section displaying site management actions, some of them were initially in the user's drop down menu, which now only have a 'manage' link with regards to management actions * stop displaying 'system' and 'schema' entities by default, only 'application' entities * put 'gc', 'registry' and 'info' views as tabs of a generic 'server information' view

[datafeed] add parser to import cubicweb xml this parser is configurable through a mapping, which basically tells which entity's relation should be copied. Multiple actions to control backport of data are available: * 'copy': copy the external entities locally (though marked as coming from the data feed source) * 'link': find similar entity internaly (e.g. for states) * 'link-or-create': try to find a similar entity internaly, and create it if necessary (won't be marked as coming from the data feed source, e.g. for tags)