feat: use grafana-toolkit to build a plugin

- fix linter warnings
- use yarn: remove Gruntfile.js and package-lock.json

++

src/styles/_variables.dark.scss

@@ -0,0 +1,9 @@
+// Dark variables
+$text-color: #d8d9da; // $gray-4
+$text-color-weak: #8e8e8e; // $gray-2
+$graph-tooltip-bg: #141414; // $dark-1
+$warning-bg: #333333; // $dark-4
+
+$theme-name: dark;
+$white: #fff;
+$red: #d44a3a;