add new env vars MM_MODULES_DIR and MM_CUSTOMCSS_FILE … (#3530)

… for setting these things from outside (and overriding corresponding
config.js properties `config.foreignModulesDir` and `customCss`)

- remove elements from index.html when loading script or stylesheet
files fails
- removed `config.paths.vendor` (could never work because `vendor` is
hardcoded in `index.html`) and renamed `config.paths.modules` to
`config.foreignModulesDir`. The `config.paths. ...` properties were
implemented in the initial commit in `js/defaults.js` but were never
functional.
- fixes `app.js` which didn't respect `config.paths.modules` before
- as `modules/defaults` is directly set in many places in the source
code restrict `config.paths.modules` to foreign modules (it has never
worked for default modules), now renamed to `config.foreignModulesDir`
- adds new `/env` section in `server.js` for getting the new env vars in
the browser
- fixes TODO in `server.js` so test directories are now only published
when running tests

These changes allow changing some main paths from outside mm with the
new env vars. You now **can** put all user stuff into one directory,
e.g. the `config` dir:
- `config.js` as before
- `custom.css`
- foreign modules

This would simplify other setups e.g. the docker setup. At the moment we
have to deal with 3 directories where 2 of them (`modules` and `css`)
contains mixed stuff, which means mm owned files and user files. This
can now simplified and leads to cleaner setups (if wanted).

js/defaults.js

@@ -19,6 +19,7 @@ const defaults = {
 	units: "metric",
 	zoom: 1,
 	customCss: "css/custom.css",
+	foreignModulesDir: "modules",
 	// httpHeaders used by helmet, see https://helmetjs.github.io/. You can add other/more object values by overriding this in config.js,
 	// e.g. you need to add `frameguard: false` for embedding MagicMirror in another website, see https://github.com/MagicMirrorOrg/MagicMirror/issues/2847
 	httpHeaders: { contentSecurityPolicy: false, crossOriginOpenerPolicy: false, crossOriginEmbedderPolicy: false, crossOriginResourcePolicy: false, originAgentCluster: false },
@@ -72,12 +73,7 @@ const defaults = {
 				text: "www.michaelteeuw.nl"
 			}
 		}
-	],
-
-	paths: {
-		modules: "modules",
-		vendor: "vendor"
-	}
+	]
 };
 
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