Skip migration of annotations for custom mapped comments or custom annotation classes

Currently, only the default annotations are processed in the migration that aims to merge annotations with comments. If a custom annotation class was used, the query did not get its objects and only processed existing objects for the default class. This was a bit ambiguous, so we will add an explicit check for custom annotation classes and skip the migration.

Additionally, we did not consider the case when the comments are using a custom mapping as this causes the migration to fail.

To work around this problem, we are simply checking before the migration if there exists a custom mapping for the comments class or if a custom annotation class is used and we skip the migration entirely.

After the whole migration process is finished and the wiki starts, the admin must make sure that 'Administration > Look & Feel > Page Elements > Document metadata visibility > Show document annotations' is set to 'Yes' so that the 'Annotations' docextra tab is still displaying the existing (not-migrated and/or custom) annotations separately from the 'Comments' tab.