Add option to embed images from external domains

[bertpoort]

Mantis id 0017997

[bertpoort]

While browsing through the Scrutinizer output I noticed a typo on line 18: $destionationemail should be $destinationemail. The rest of the file uses the correct $destinationemail.

[michield]

Great, as you put a flag around it "EMBEDEXTERNALIMAGES", I will merge and then we can take it from there. I will probably leave it off for the time being and we can see if we can get some feedback from people.

[michield]

https://resources.phplist.com/system/config/embedexternalimages

[bertpoort]

Great. When it's ready for production use it probably could use some extra configuration parameters and possibly better default values for the following:

• Max age for cached file (currently 1 day)
• Max size for cached file (currently 1 MB)

Also note that in a worst case scenario this could pause sending of the first message for 30 seconds per external image since that's what the HTTP timeout values are set to. Errors are cached so only the first message would be affected.