feat: use grafana-toolkit to build a plugin

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

++

src/styles/_variables.light.scss

@@ -0,0 +1,10 @@
+// Light variables
+$text-color: #52545c; // $gray-1
+$text-color-weak: #767980; // $gray-2
+$graph-tooltip-bg: #dde4ed; // $gray-5
+$warning-bg: #e9edf2; // $gray-6
+
+$theme-name: light;
+$white: #fff;
+$red: #d44939;
+