Member phone validator

[b-d055]

Attempt to address #374

[Andrew-Dickinson]

Hey @b-d055. Thanks for your contribution. Sorry we didn't look at this earlier!

This change looks good to me, but we should have a few tests just to confirm it's behaving correctly. Could you add a unit test for:

1. Basic validation that creating a new model object and doing model.save() works with a valid phone number but fails with an invalid one
2. Submitting the join form with an invalid phone number throws an error
3. Attempting to change the phone number via the API fails with an invalid number

[Andrew-Dickinson]

Also be sure to rebase onto main, we've made some linting changes so you'll want to double check that invoke lint is working as well

[Andrew-Dickinson]

Closing this in favor of #450