Add empty `ab` element after each excerpt's text node to hold the `@source` attribute

[wlpotter]

To fix https://github.com/srophe/caesarea/issues/348 we need a place after the excerpt's texts but before the tei:note elements.

We'll use a tei:ab element and move the @source attribute from the containing tei:ab element for the corresponding bibl.

• [x] fix in current data (should wait on #137 so we don't retain incorrect source attrs). Will need to test if there are note elemetns
• [x] update the post-processor to put this in the correct place for new records

[wlpotter]

Note to self, per #137 make sure the generation of the footnote ab takes into account if there is a translation bibl or not. Test whether //body/listBibl[1]/bibl[2] is non-empty and only add the @source attribute if it is non-empty.

[wlpotter]

Added this to current data. Leaving this issue open to check how the HTML looks.

I also want to troubleshoot a bit as I had to do some after-the-fact regex find and replace to remove whitespace....This might just be what it's going to be, but if it is the case that the post-processor doesn't add the whitespace, then I won't worry about this.

[wlpotter]

One other thing to note here is that the main TEI schema doesn't like the idea of nesting ab elements, so it's currently not validating. We can customize our schema or pick another element (like ref or ptr).