transcriptions can be orphaned or lost when merging records

[kseniaryzhova]

Describe the bug
Two PGPIDs (799 and 801) had two transcriptions for the same document (one from Gil, the other from Goitein). When merging the two records in production, Goitein's transcription did not merge properly. The document detail page was showing that there were two transcriptions, but Goitein's had no text in either the editor or the display.

Expected behavior
When merging two records for the same document, I want everything from both records (including transcriptions) to be present in the newly-merged PGPID.

Additional context
@rlskoeser fixed this by going into the python console and updating the manifest ID to point to the right document BUT we will need this feature for future data cleanup.

dev notes

Logic probably needs to go in Document._merge_footnotes — if any of the footnotes being merged are digital editions with transcription content, the code needs to look for corresponding annotations and update the manifest id in the record. (Right now annotations are related via uris and not foreign keys, but we may want to revisit that — should keep the merge code in mind when we do, since the logic could be impacted by this change.)

[kseniaryzhova]

@rlskoeser it works really well! Woo! Thank you!