Offer different email frequency in onboarding signup

[ilicfilip]

Context

Offer users to sign up for monthly or quarterly emails (instead of just weekly).

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

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[ilicfilip]

This is implemented on the user (plugin) side:

There is PR on the server side as well, user selection will be saved but there are a few things that needs to be decided:

• Weekly, monthly and quarterly emails will be different - what is their content?
• If content is different, do we need to collect more (different?) data from the REST API endpoint?

Once that is decided, the server side should be adjusted.