[js form validation] fix case where we've None/null keys in the errors dictionnary

[cleanup] exists imported, use it

[form] CompoundField should not return itself in actual_fields, this avoid error on iliane with 3.7.2 where it's marked as required (from the schema) while value are provided by subfields

[repo] we want a hooks manager and hooks initialized event on quick start (we trigger events such as server_backup / server_restore for instance)

[distribution] sql extensions files now have proper extension, fix manifest file

backport stable

[events manager] onevent decorator may be given extra arguments to give to em.bind

[cleanup] some notes

[reload] on registry reloading, we should change class of users of opened session

[appobject imports] don't include __init__ in package's name. See comment for more info

[rset] cubes such as iliane hi-jack rset description by poping some lines from it. Handle this on RepeatList

[undo] basic support for undoing of entity creation / relation addition

[undo] init entity cache when undoing an entity deletion

[undo] consistent is/is_instance_of processing

[repo] don't need rset description on those queries