New: autoload to specific manuscripts

-Reinstated "Cantus MEI Editor is loading" text.

-Removed window.onload as its current existence was unnecessary.

-Manuscripts now load based off the siglum_slug being added to the URL; no slug triggers the "choose a manuscript" overlay.

-Dropdown in Diva-Manager plugin to switch manuscripts.
Merge branch 'master' of https://github.com/DDMAL/cantus