[ready] invitation flow easier for new users

**CR**: :white_check_mark: (mix)

**QA**: :white_check_mark: 

> pull down locally, check app loads :white_check_mark: 

> *test core functionality of feature:*
>  accept an invitation & create account with form email :white_check_mark: 
>  accept an invitation & create account via SSO :question: - I did see it sending to the right API but my local server isn't set up to work with fb. might need trying on clone
 
> *test functionality adjacent to this feature:*
> didn't test request membership arc :ok: / :question:  - I think this is ok
> create and account from scratch :white_check_mark:  
> start a group :white_check_mark: 

> public/ private functionality :ok: NA
> logged out functionality :ok: NA

> test in IE 8,9 :ok: NA
> test in production environment :question: 

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**CO signoff**: :question:
**DO signoff**: :question:

**PO signoff**: :question: