[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)

[xml] enhance default entity xml view * format attributes according to their type in a predictable way * move eid/cwuri as top level tag attributes * control which relations should be in using 'relation' form param

[pyro source] benefit from addition of latest_retrieval on CWSource to avoid using a CWProperty to store pyrorql source latest synchronization.

[repository] #1460066: backport datafeed cube as cubicweb source * add some attributes to CWSource to handle this kind of source (not natural to put everything in 'config' string). Adding a CWSource subclass has been attempted then rollbacked because it adds pain to handle multi-sources planning and it introduce an ambiguity on a generic relation (cw_source), which may be a penalty in multiple case * data feed sources are a new kind of source, namely 'copy based', which have no effect on the query planner * a data feed source is associated to a list of url and a parser (appobjects in the 'parsers' registry * entities imported by a data feed have cwuri set to their url on the distant site, their cw_source relation point to the data feed source, though their source stored in the entities table (returned by cw_metainformation) is their physical source, hence 'system'

backport stable

[entity, metadata] huuum, use resolvable url as cwuri... And fix existing ones.

[doc] update some copyrights

[repo, ms] fix pb introduced in 6941:9ed02daa7dbb resulting in missing relation deletes when an entity is deleted