[READY] add event sequence id to identify sequence within discussions

#### Note: after this is merged we need to run the data migration rake task

:white_check_mark: :  **CR** 

:white_check_mark:  : **QA** 

> :white_check_mark: pull down locally, check app loads

> *test core functionality of feature:*
> :white_check_mark: liking
> :white_check_mark: unliking

> *interact with everything in proximity of the core feature:*
> :white_check_mark: posting comments
> :white_check_mark: edit discussion title
> :white_check_mark: start new discussion  

> :white_check_mark: logged out functionality
> :ok: public/ private functionality- NA

> :ok: test in IE 8,9 - no changes relevant to IE
> :ok: test in production environment - NA

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:white_check_mark: : **CO signoff** 
:ok: : **DO signoff** - nothing changed in interface

:question: : **PO signoff**

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useful symbols :white_check_mark: :ok: :x: :question: