Fixes migration for mixed column contents

If a column is blank it stays blank. If the column is not blank it is
handled as usual.
Adds changelog entry
Adds migration for former deleted PEs
Adds changelog entry
Handles missing ids
Merge pull request #716 from opf/fix/query_migration_for_mixed_column_content

[FIX] Query migration for mixed column contents
Deletes work package journal entries
Removes references to 'deleted_at' column
Fixes custom field assignment
Changes event data structure
Adds work package event properties
Fills user/project references with one query
Adds provider reference to event
Moves work package  activity provider settings
Moves changeset activity provider settings
Moves news activity provider settings
Moves wiki content activity provider settings
Moves message activity provider settings
Adds helper method
Moves time entry activity provider settings