Domain specific email validation flag

[chriso]

#832 added additional validation for gmail addresses:

• certain characters are not allowed, for example _
• the user part must be 6 to 30 chars inclusive

Example of gmail rejecting a short username:

Unfortunately this has caused some confusion upstream where test fixtures would use a syntactically valid email like test@gmail.com only to have it fail validation.

This PR updates the validator to skip domain specific validation unless a new domain_specific_validation flag is set. The flag is disabled by default.

This fixes #853 and #865

[patelnav]

Loving how responsive and quick you guys are with updates. Awesome job 👌🏾