Email newsletter sign up forms for sidebar ads

[relyks]

Description

This PR adds the ability for newsletter sign up forms to be added to sidebar ads and for specific newsletter mailing lists to be chosen.

Code Changes

• Adds the option to include additional newsletter mailing lists as an array of strings representing each newsletter to the newsletter sign up form
• Modifies the utility function for subscribing newsletters to include an optional parameter for those additional newsletter mailing lists.
• Retrieves newsletter mailing lists that may be part of a sidebar ad from Strapi.
• Removes an old utility function used for subscribing to NationBuilder which is no longer in use.

Notes

• The parts related to Strapi are dependent on a code update for Strapi to be usable in production.
• An option to include specific newsletters for a locale such as Hebrew needs to be added in the future in another PR.
• Code to show the newsletter sign up form as a sole button instead of a form for logged in users will be done in a future PR.

[edamboritz]

1. The relationship between this PR and Update subscribe endpoint for multiple newsletter subscriptions #2141 is not entirely clear. Where are the available newsletters being stored?
2. Does the information about what newsletters the user is signing up for actually get rendered to screen anywhere?

[relyks]

The relationship between this PR and Update subscribe endpoint for multiple newsletter subscriptions #2141 is not entirely clear. Where are the available newsletters being stored?

The available newsletter mailing lists are stored in Salesforce. I made a public API endpoint on the Sefaria side in that PR to retrieve an array of those lists. Strapi will utilize the endpoint to fetch the available mailing lists for people creating sidebar ads. It'll allow them to select which lists users will be subscribed to when those users submit their email addresses. Engineers will also be able to use the endpoint to view the available mailing lists and hard-code specific newsletter subscriptions for their sign-up scenarios. I haven't pushed the changes to the Strapi repo yet.

Does the information about what newsletters the user is signing up for actually get rendered to screen anywhere?

Only in Strapi right now. I'm also being proactive. MarCom wants to be able to show the available mailing lists in a user's settings and let them subscribe/unsubscribe from there.