fix: Use correct flag for MDN sent and don't show junk flag used by Thunderbird as tag

[DerDreschner]

This is a follow-up PR from #12187 and fixes the following points that were discovered during development there:

• Fix: Use correct flag for MDNSent throughout the code base (mdnsent -> $mdnsent - $ sign was missing). There is no case known to me where this flag is being used without the $ sign (even Thunderbird does it, I've tested that).
• Fix: Thunderbird uses the non-standard junk flag without $ sign, which was previously shown to the user as tag. This is being fixed here.

[ChristophWurst]

Looks good! Thanks :)

We'll give this a test soon (I'm looking at @kesselb 👀)

[kesselb]

Thanks for your pr 👍

Please add a mapping, similar to $junk, at:

mail/lib/Db/Message.php

Lines 284 to 295 in 6de0c2a

	if ($flag === Tag::LABEL_IMPORTANT) {
	$this->setFlagImportant($value);
	} elseif ($flag === '$junk') {
	$this->setFlagJunk($value);
	} elseif ($flag === '$notjunk') {
	$this->setFlagNotjunk($value);
	} else {
	$this->setter(
	$this->columnToProperty("flag_$flag"),
	[$value]
	);
	}

It's currently using the else block, but the database column is named flag_mdnsent and thus, setting flag_$mdnsent throws an exception.

[kesselb]

If we change it there, we should also change for consistency:

mail/src/components/MdnRequest.vue

Lines 53 to 55 in 6de0c2a

	mdnSent() {
	return this.message.flags.mdnsent
	},

mail/lib/Model/IMAPMessage.php

Line 165 in 6de0c2a

'mdnsent' => in_array(Horde_Imap_Client::FLAG_MDNSENT, $this->flags, true),