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/server_functions.js

@@ -128,4 +128,30 @@ function getVersion (req, res) {
 	res.send(global.version);
 }
 
-module.exports = { cors, getConfig, getHtml, getVersion, getStartup };
+/**
+ * Gets environment variables needed in the browser.
+ * @returns {object} environment variables key: values
+ */
+function getEnvVarsAsObj () {
+	const obj = { modulesDir: `${config.foreignModulesDir}`, customCss: `${config.customCss}` };
+	if (process.env.MM_MODULES_DIR) {
+		obj.modulesDir = process.env.MM_MODULES_DIR.replace(`${global.root_path}/`, "");
+	}
+	if (process.env.MM_CUSTOMCSS_FILE) {
+		obj.customCss = process.env.MM_CUSTOMCSS_FILE.replace(`${global.root_path}/`, "");
+	}
+
+	return obj;
+}
+
+/**
+ * Gets environment variables needed in the browser.
+ * @param {Request} req - the request
+ * @param {Response} res - the result
+ */
+function getEnvVars (req, res) {
+	const obj = getEnvVarsAsObj();
+	res.send(obj);
+}
+
+module.exports = { cors, getConfig, getHtml, getVersion, getStartup, getEnvVars, getEnvVarsAsObj };