past institution identifiers/notes

[annamorphism]

I decided to make https://cantusdatabase.org/institution/26 have the correct name/siglum associated with Rare Books (which is where all of Cantus's current sources at McGill live.) I think it would be good to have a public note to the effect that these used to be labelled CDN-Mlr (just in case they are cited somewhere). It's not exactly a "past identifier" for the institution, though, since McLennan still exists--just a past identifier that we used erroneously. I suppose I could go into every McGill record and note that it might have been referred to as "CDN-Mlr MS ..." in the past, but it seems both faster and more accurate to do so at the institution level.