* Adding annotations to backend
* Auto fetching Annotations on the backend
* Closing the loop
* Adding missing files
* annotation layers UI
for https://github.com/apache/incubator-superset/issues/3502
* a few fixes per code review.
- add annotation input sanity check before add and before update.
- make SelectAsyncControl component statelesis, and generic
- add annotation description in d3 tool tip
- use less variable to replace hard-coded color
