[MlMt] Domain whitelist for remote content, revisited

[Sam Hathaway]

Hi MailMate Users,

A while ago I posted asking if there was a way to tell MailMate to load 
remote content only from specific domains. The rationale here is that, 
since sender addresses can be easily spoofed, allowing remote content 
loading based on sender address is not really effective at preventing 
tracking. By allowing loads by source domain, we can effectively control 
where potential tracking data ends up. (This is one of the few features 
I miss from Thunderbird.)

Folks suggested that I try using Little Snitch to do this, and so I did. 
Turns out, Little Snitch is too intrusive when used this way, as it asks 
about each domain in turn, and I have to allow or deny each one. (And 
there are *lots* of domains referenced in your average HTML email!)

Thunderbird, in the other hand, uses a popup menu which lists each 
domain and allows the user to select which they would like to allow 
content from.

Screenshot here:
https://support.mozilla.org/legacyfs/online/sumo-media/gallery/images/2016-06-25-09-51-48-ba4a40.png

Docs here:
https://support.mozilla.org/t5/Privacy-and-security-settings/Remote-Content-in-Messages/ta-p/21655

So I guess this boils down to a feature request. How about it, Benny?
-sam