[migration] only define variable if needed

[migration] handle non-existing attribute type when adding an entity type The final type needs to be added to the schema first. Related to #7569998.

[migration] handle the case where new final entity type is not yet in the schema This may occur on new cube addition. Related to #7569998.

[migration] config.add_cubes should ensure site_cubicweb of new cubes is called Related to #7569998

[migration] test addition of a cube providing a custom final type This does not work yet. Related to #7569998

[server] add test demonstrating cache update pb since cw > 3.21 Fixed by the previous changeset.

Bring back the separate web-side entity cache Prior to changeset 635cfac73d28 "[repoapi] fold ClientConnection into Connection", we had two entity caches: one on the client/dbapi/web side, and one on the server/repo side. This is a waste, but it is actually needed as long as we have the magic _cw attribute on entities which must sometimes be a request and sometimes a cnx. Removing the duplication caused weird problems with entity._cw alternating between both types of objects, which is unexpected by both the repo and the web sides. We add an entity cache on ConnectionCubicWebRequestBase, separate from the Connection's, and bring back the _cw_update_attr_cache/dont-cache-attrs mechanism, to let the server/edition code let caches know which attributes have been modified Entity.as_rset can be cached again, as ResultSet no longer modifies the entity when fetching it from a cache. Contrary to the pre-3.21 code, _cw_update_attr_cache now handles web requests and connections in the same way (otherwise the cache ends up with wrong values if a hook modifies attributes), but dont-cache-attrs is never set for (inlined) relations. Closes #6863543

Backout "[entity,storage] remove hackish code from the bfss path The merge of the repo-side and web-side entity caches doesn't actually work, because entities have a _cw attribute which is sometimes a connection and sometimes a web request, and whose nature changes not-really-predictably whenever ResultSet.get_entity() is called and brings it out of the cache. This reverts commit 59a79300f213.

merge with 3.21.5-2

[doc] add documentation about drop_cube

Added tag debian/3.21.5-2 for changeset 228b6d2777e4

[pkg] don't install dataimport/test in both cubicweb-server and cubicweb-dev Closes #9239572

merge with 3.21.5

Added tag 3.21.5, debian/3.21.5-1, centos/3.21.5-1 for changeset e0572a786e6b

[pkg] 3.21.5

[js] Fix a ReferenceError in cubicweb.preferences.js Rename function using proper English along the way. Closes #8864731.