new feature: search/create repetenda field

[annamorphism]

From an email:
"Some researchers would really like to have the refrain of chants indicated in a clear way for the types of chants that are responsorial.
The cue word for the refrain is often in the manuscripts, and it can give us ideas about provenance and transmission.
There wasn't space for this in the earliest Cantus inventories, so they just didn't record them.
In the mid-2010s, we started putting the refrain 'cue word' (the repetendum, pl. is repetenda) at the end of the 2nd text field, after a | vertical line.
Being able to compare the repetenda among Office responsories, for example, would be very interesting and it may give us a better idea of performance traditions in different places."

and from a later email:
"One issue to be aware of and to address simultaneously (and capture in the GitHub issue so we don’t forget): if there is a Volpiano transcription that repetendum cue is also transcribed musically and it is used in the auto-alignment of music and text.
So do we want both removed from the display? Or is the display going to grab information from different fields?"

[annamorphism]

@dchiller proposed that much of what we want might be accomplishable simply by searching for words after the pipe, and Debra agreed that that might be easier.
(The concern from the second email is what would happen if one were to spin off the repetenda into their own entries, but I think that is more complicated than creating a search that lets you find "examples of this responsory where the repeat after the verse starts "Ave...."").