cubicweb #1931543 Refactor __linkto [resolved]

__linkto is now partly handled by entities although it is really form specific; its present behaviour (notably remove sometimes __linkto keys from request form) is coded in entities but is due to form handling; the code in is thus difficult to understand, and would be better refactored into a base form class.

done in3.14.0
load left0.000
closed by#65e460690139 [form, entity] refactor '__linkto', now handled by the entity form, not the entity itself. Closes #1931543