Member Profile: Update regex validation on Mobile Phone field to accept only numbers

[mcookWI]

Enhancement requested in #287 to update regex validation to have Mobile Phone field on the Member Profile to only accept numbers. Right now, validation on the field requires a format such as +10001112222, meaning a phone number entered as +12 would be considered valid, while (414)555-1212 or even 4145551212 would not be.

Phone numbers can be a maximum of 15 digits, including country code.