fix: improve MX and SPF domain handling

[cquintana92]

⚠️ Please review with attention ⚠️

This PR performs the following changes:

1. Move out logic from the dns.html template file into the controller, passing the values to be used instead of calculating them in the template.
2. Offer new functions that return the expected MX and SPF records on the CustomDomainValidation class.
3. Make use of the aforementioned functions during the MX and SPF verification process.
4. Encapsulate the MX records into a MxRecord class instead of a Tuple[int, str].
5. Adapt the tests accordingly to these changes and add some new ones.