Allow list of test phone numbers that will not be rate limited.

[shane-tomlinson]

Right now the content-server has a bunch of SMS related functional tests that aren't run on "prod-like" environments because they send actual SMSs. Instead of using randomly generated phone numbers, I tried using test phone numbers provided by phone companies, but was soon rate limited.

We should allow SMS messages to be sent to these special phone numbers w/o being rate limited, perhaps keeping a list of numbers or regexps that aren't rate limited - sort of like we do with email addresses.

Ref mozilla/fxa-content-server#5689
Replaces mozilla/fxa-auth-server#2220