Beautify Changelog for 0.79 (facebook#49987)

Summary:
Pull Request resolved: facebook#49987

I went through the whole changelog for 0.79 and applied categories to all the entries + did some significant cleanup (remove some entries that were actually internals, grouped and sorted various items)..

Changelog:
[Internal] [Changed] -

Reviewed By: cipolleschi

Differential Revision: D71003373

fbshipit-source-id: 5a95e619f1cf2ffc84263b92104efee16fd74e53

Author: cortinico