feat: add backspace and caps lock icons (#803)

Summary:
- svg sprite generate with xmlns:xlink="http://www.w3.org/1999/xlink" namespace
- uhk-icon-agent-icon css class renamed to uhk-icon-pure-agent-icon because
  it collided with agent icon sprites
- added backspace and caps lock icons

package.json

@@ -67,7 +67,7 @@
     "rimraf": "2.6.1",
     "standard-version": "4.2.0",
     "stylelint": "9.5.0",
-    "svg-sprite": "1.4.0",
+    "svg-sprite": "1.5.0",
     "ts-loader": "2.3.1",
     "ts-node": "7.0.1",
     "tslint": "5.9.1",