Support for displaying multimedia messages in Slack & Facebook (#28)

[aarora91]

• First implementation of Phase A: Conversation Message Contract

• Modified normalize to work with Dave's multi-post action

• small multiple-post fixes

• add unit test

• move to facebook package

• new post sequence

• Modified logic for displaying quick replies and generic template buttons

• fix lint warnings

• add new tests

• fix fb json

• Fixed unit tests as per the new implementation

• Fixed ITs and e2e tests for normalizer

• Fixed int and e2e tests for Slack and Fb

• fix for broken multiple-post UTs

• Added blocking statement to UTs

• Code refactoring + cleanup

[dgterry]

Going to close this and create a new version of it after I cleanup develop a bit.