Add source details column to signups table

[sbsmith86]

FEATURE OVERVIEW

User Story

To be able to track more details about an sms signups, add a signup_details column to the signups table that can hold things like broadcast ids, etc.

Open questions:

Should this be implemented the same way it is done in other systems?

What are the set of values we want to store in there? (ask aaron, sergii)

[ngjo]

As a developer, I want to add source details to to the signups table so that we can get some meta data around the signup

[chloealee]

@ngjo I think we can close this issue and the card bc we now have this in the signups table!

[chloealee]

well we have source_details but maybe we don't want to close because it doesn't seem to be populated with the data described in the card...