Adds migration for legacy journals.

Legacy journals need to be transformed to current journals. This
amazing new migration does it.
Check migration prerequisites.
Adds journal creation to migration.
Extend legacy journal migration

The legacy journal migration now includes normalization into the data
tables. This involves combining previous journals (diffs) into the new
normalized schema and extends to changesets, news, messages and work
packages.
Introduces new migration dated after previous fixes.
Remvoes obsolete migration.
Adds support for more journals.

Now also migrated wiki_content_journals as well as time_entry_journals.
Also, wikis now have some custom behaviour for data -> text conversion.
Adds support for lock_version in wikis.
Adds support for work package attachments and custom fields.

Those are ignored for now.