client: raise nodejs requirement to v12.16.1 * Raise nodejs requirement to v12.16.1, because v8 is unmaintained now. And even if nodejs is just used for building the single page web application it should be security maintained, e.g. for being able to get the right modules when downloading. * Add check of node version to packages.json in engines section. This can be easily circumvented if someone really wants to try building with a different version, e.g. v10, but most tests will be done with v12 from now on, so it is better to have this check in there. And v12.16.1. is the current stable version, so it should be the minimum.

--- a/client/README.md	Tue Apr 07 15:16:11 2020 +0200
+++ b/client/README.md	Tue Apr 07 16:34:35 2020 +0200
@@ -1,7 +1,7 @@
 * Install dependencies
 
-  * [Yarn](https://yarnpkg.com/) and a compatible nodejs version (>=8.10.0)
-    is needed.
+  * [Yarn](https://yarnpkg.com/) and a compatible nodejs version
+    is needed, see `engines` section in `packages.json`.
 
   * Install (`xgettext`) tool (e.g. for Debian xgettext (GNU gettext-tools) 0.19.8.1)