Fix permissions for webconf invitations

Was blocking non-admins from sending invitations. Now users can send invitations
in their personal room or in rooms that belongs to spaces they are members of.

refs #1179
Always fall back to local authentication if LDAP fails

Some errors in the authentication via LDAP were still forcing an error that
made the authentication stop, not falling back to the local authentication.
Now all possible errors will fail only the LDAP strategy, always falling back
to the local authentication. All errors are also logged.

refs #1055