[migration] closes #1777411: schema modification commands shouldn't crash if already exists

[rqlrewrite] closes #1772135: 1. don't try to reuse a relation from another statement (eg because a subquery has been introduced) 2. _use_orig_term should consider the current statement

test fixes

[datafeed cw parser] refactor: split logic that was in the parser into: * an "item builder" component, turning an etree xml node into a specific python dictionnary representing an entity * "action" components, selected given an entity, a relation and its role in the relation, and responsible to link the entity to given related items (eg dictionnary) This changes make it easy to add new action or to override existing one's behaviour according to the entity, the relation, etc... Also refactor the xml view so one can also override what's included in the tag used to mark the entity as used by a relation, hopefully allowing to reduce the number of http requests needed for a full datafeed import.

[datafeed, entities] url for entities from a datafeed source should be on their origin site. Closes #1769391

cleanup