Subscription Limits

[tylersamples]

Add configuration entry subscribe_limit and upon subscribing query by the "From" phone number to check that the total does not exceed what is configured. subscribe_limit should default to 0 and by default should not limit the amount of subscriptions a single phone number can have.

Upon completion add the following to the features list under "Multiple Subscriptions"

ExCourtbot can be configured have an upper limit of subscriptions per phone number.