This change:
- Adds the AddNamespaceAncestryIdsToIssuesMapping migration
- Adds the RedoBackfillNamespaceAncestryIdsForIssues migration
- Changes the namespace_ancestry method
method to have trailing separator, which is needed to get all documents
belonging to one namespace in 1 query. Without this last separator
`namespace_ancestry_ids = '1-2'` will also match `1-233` and return
incorrect results.
- Extracts common examples for adding mappings and backfilling a new
field into migration shared examples (migration_shared_examples.rb)
and uses them for migration specs.

EE: true
Changelog: changed