Make Notifications generic and decoupled from Invitations

[dborovcanin]

@dborovcanin
Just saying my thoughts

Having stream topic as constant make notification service works only for invitations
We need to make this configurable via file

The file should have event topic,event payload parser, notification condition check , then emailer or slacker or webhooker

Originally posted by @arvindh123 in #3254 (comment)