[feature] Better loading indicator

- Use react-topbar-progress-indicator as a global loading indicator.
- Added a global reducer that only holds `isFetching` for now.
- Display loading indicator on any `*_REQUEST` actions.
- Closes #103

yarn.lock

@@ -6811,6 +6811,12 @@ react-toolbox@^1.2.1:
     normalize.css "~5.0.0"
     react-css-themr "~1.4.1"
 
+react-topbar-progress-indicator:
+  version "1.0.0"
+  resolved "https://registry.yarnpkg.com/react-topbar-progress-indicator/-/react-topbar-progress-indicator-1.0.0.tgz#3379ab2cb840c1bc1bc22fb5fc871688b9104dd4"
+  dependencies:
+    topbar "^0.1.3"
+
 react-waypoint@^3.1.3:
   version "3.1.3"
   resolved "https://registry.yarnpkg.com/react-waypoint/-/react-waypoint-3.1.3.tgz#1101fb8a27556a199150c7bfd34428606b5fc7e4"
@@ -8043,6 +8049,10 @@ to-fast-properties@^1.0.1:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/to-fast-properties/-/to-fast-properties-1.0.2.tgz#f3f5c0c3ba7299a7ef99427e44633257ade43320"
 
+topbar@^0.1.3:
+  version "0.1.3"
+  resolved "https://registry.yarnpkg.com/topbar/-/topbar-0.1.3.tgz#c9ef8776dc4469f7840e6416f4136ddeccf4b7c6"
+
 tough-cookie@^2.3.1, tough-cookie@~2.3.0:
   version "2.3.1"
   resolved "https://registry.yarnpkg.com/tough-cookie/-/tough-cookie-2.3.1.tgz#99c77dfbb7d804249e8a299d4cb0fd81fef083fd"