[repository] Use an internal connection in register_user

avoid to internally raise 3.19 warnings

[sources] ensure we have a cnxset in undoable_transactions

[server] move security/integrity hook management away from InternalSession.__init__ and to Repository.internal_{session,cnx} instead. Lets internal_cnx avoid deprecation warnings.

[migration] make 'session' object be a server-side Connection, not ClientConnection I think this better corresponds to the previous behaviour. However, that Connection does not have a cnxset, which should probably be changed (for both interactive_shell and cmd_process_script).

[sources] Skip problematic sources when starting shell instead of crashing. Closes #3670208 At least this gives an opportunity to fix the problem in using c-c shell, which is currently not possible because it crashes during initialization. Notice that since sources are copy based, it should not be a problem at all to have some disabled, beside for services such as authentication sources as ldapfeed.

[web] fix language negotiation Regression caused by commit 6fd0ac6506cb "[web-request] handle default language earlier". We would handle language negotiation at request init time, but overwrite the selected language with the site default when calling set_cnx.

[merge] bring the 3.17.14 fixes to 3.18

[test/utils] repair concat-resources test That was forgotten in 240a620b9cd3. Related to #3670503.

[pkg] bump cubicweb.spec