Initial copy of files from the riot-web repo

None of this will work as it will need tweaking (at the very least
I've not copied the origin migrator because that's had long enough)
but these are files which already existed in their current state and
so don't need re-reviewing.

build/linux/after-install.tpl

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+# Link to the binary
+ln -sf '/opt/${productFilename}/${executable}' '/usr/bin/${executable}'
+
+# SUID chrome-sandbox for Electron 5+
+# Remove this entire file (after-install.tpl) and remove the reference in
+# package.json once this change has been upstreamed so we go back to the copy
+# from upstream.
+# https://github.com/electron-userland/electron-builder/pull/4163
+chmod 4755 '/opt/${productFilename}/chrome-sandbox' || true
+
+update-mime-database /usr/share/mime || true
+update-desktop-database /usr/share/applications || true