ZBUG-3629: fixed handling of image in signature

[k-kato]

Issue:
An inline image is not included in a reply/forward message when the sender of the original message is added to trusted address.

Root cause:
Source of an image was fetched in the following priority:
dfsrc => data-mce-src => src

Before the fix, data-mce-src is set for an image in a signature.
<img pnsrc="cid:edf162835aead51e0a185735d5add806ee0b8710@zimbra" data-mce-src="https://HOST/home/ACCOUNT/Briefcase/imageg.png" src="https://HOST/service/home/~/?auth=co&amp;loc=en_US&amp;id=400&amp;part=2.2">

At reply/forward, the url in data-mce-src is set in src of img tag when a trusted address is configured.

ZmComposeView.js, line 1239:
else if (showImages) {
	var src = dfsrc;//x + "&t=" + (new Date()).getTime();
	num++;
	images[i].src = src;
	images[i].setAttribute("dfsrc", dfsrc);
}

Then the image in the forwarded message points the briefcase path. If it is not shared, the image cannot be fetched.
The full access path to a briefcase file is unnecessary for a recipient. The image file is embedded in a message source and referred as cid. The image should always be fetched from the embedded data.

Fix:

• When a signature is set in Compose page, data-mce-src is removed from the image.
• When a signature image is reused at reply/forward, cid in src is used if it exists.