Immutable public and global content management (#5390)

* Use custom init script instead of postinstall

* Revert changes to start scripts in src\electron

* Add global data to content manager

* Add migration for public overrides and user.css location update

* Update npm publish workflow to use 'omit=dev' flag in npm ci commands

* Rename user.css readme file

* Fix indentation in userCssMiddleware function

* Add directory creation for content target

* Restore template compile location

* Move stylesheet up in index.json

* Use path.resolve for user.css file path in userCssMiddleware

* Correct capitalization in "Not Found" error page title and heading

* Remove init run from startup scripts

* Simplify user CSS file path resolution

* Update userCssMiddleware comment

public/css/!USER-CSS-README.md

@@ -0,0 +1,7 @@
+# Looking for user.css?
+
+user.css is now located under your data root directory in the "_css" folder.
+
+Example for the default data root:
+
+/data/_css/user.css