Modify XBlock root class's index_dictionary method to return common fields e.g `title` and `content_type` by default.
*Merge deadline*: "None"
*Author notes and concerns*:
1. I am not sure what kind of unit test should I write to test this feature as all the real XBlocks lives in other repos. So, if you have some idea, please discuss.
[ ] @xitij2000
[ ] @davestgermain