Add auto opt in feature

[engelhartrueben]

Fixes # (issue)

Add auto opt in feature

Description

The feature works similar to how we opt people out.
Spoke checks if the contacts reply contains opt in language. The default language is "START" (case sensitive).
After a reply is marked as an opt in, it is added to the opt_in table and marked as is_opted_in as true in the campaign_contact table. This feature is also set up to respect opt ins by organization.

The ideal scenario is texting a list from the voter file, and end the text with "Text "START" to stay in touch with our organization" or something of the sort. When the contact texts back "START", this triggers.

Currently, this feature only gathers information. However, this is the backbone to updating VAN contacts as opted-in to text messages or adding a contact loader specifically for contacts that opted in.

This has been tested with/on:

• sqlite3, but not postgresql.
• twilio
• production environment

Checklist:

• I have manually tested my changes on desktop and mobile
• The test suite passes locally with my changes
• If my change is a UI change, I have attached a screenshot to the description section of this pull request
• [ oops ] My change is 300 lines of code or less, or has a documented reason in the description why it’s longer
• I have made any necessary changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• My PR is labeled [WIP] if it is in progress