Allow dots at end of FQDNs in DNS editor

[tbradsha]

Steps to reproduce

1. Navigate to the DNS editor.
   2a. Attempt to create a CNAME with "Alias of" set to example.com.
   2b. Attempt to create an MX record with "Handled by" set to example.com.
   2b. Attempt to create an SRV record with "Target Host" set to example.com.

What I expected

A dot should be allowed after fully-qualified domain names. See for example:
https://serverfault.com/questions/803033/should-i-append-a-dot-at-the-end-of-my-dns-urls

What happened instead

Our system claims said values are invalid:

• 2a:
  
• 2b:
  
• 2c:
  

[tbradsha]

See also #14694.

[inaikem]

Trailing periods are not permitted in Calypso but are added in the backend. A recent report can be found here: 382665-ZD.

I'll do a bit more research here and direct/re-label this issue from there.

[inaikem]

Discussing here: p2MSmN-8lK-p2

[hambai]

@klimeryk do you think we can fix this within the scope of the https://github.com/Automattic/wp-calypso/projects/107 project?

[inaikem]

Another user-report here: 29042957-hc

[aamills]

I had a confused user in 29769074-hc, regarding the trailing period on a CNAME verification record from Hubspot.