Relicense Ely Office's own code from AGPL-3.0 to MIT

Applied on top of the released v0.1.0 so the tag says MIT, as main does
(271468d). Bundled engines keep their licences: LibreOffice (MPL-2.0),
Git (GPL-2.0) and MuPDF (AGPL-3.0).

README.md

@@ -44,4 +44,4 @@ Packages: `packaging/macos/release.sh` writes each app's `.app` and `.dmg`, the
 
 ## Licence
 
-AGPL-3.0-or-later (`LICENSE`). Third-party components keep their own licences; see `docs/licenses.md`.
+MIT (`LICENSE`). Third-party components keep their own licences; see `docs/licenses.md`.