Lots of changes!

We now enforce a maximum lateness of six months for scheduled messages, i.e., we won't send scheduled messages that were scheduled to be sent more than six months ago even if the user has the relevant preference disabled or set to a higher value. This is both for safety and so that we don't have to let the cache that keeps track of which messages have already been sent keep growing forever.

Speaking of which, Send Later keeps records in internal storage of which messages have already been sent so they won't be sent again if something goes wrong in the add-on. Previously this record was being kept essentially forever and growing without bound. As noted in the previous paragraph, we now only need to save it for six months, so records older than six months are now cleaned up automatically.

Check when a message is being scheduled whether it has encryption or signing enabled, or whether drafts encryption is enabled, and if so, pop up a message warning the user and don't let them schedule the message.

Remove the internal "Autocrypt" message header (added in Thunderbird 115) from scheduled messages before they are delivered.

Because TB115 doesn't have the custom column showing the scheduled send date, it's difficult for users to see which messages are scheduled and when they're scheduled for. To work around, this put the scheduled send time in the Date field of scheduled messages. This can be disabled by setting the advanced preference scheduledDateField to false.

Sort the keys in the advanced configuration editor so it's easier to find the one you want.

When the user makes a chance to a preference in the top part of the preferences window, propagate the change into the advanced configuration editor. Otherwise, if someone opens the preferences window, makes a chance at the top, then makes a change in the advanced configuration editor and clicks Save there, the change that the user made before in the top part of the window is overwritten.

Don't force the Send Later button to be visible in the compose window toolbar. If the button isn't visible then the Send Later pop-up is an independent pop-up window rather than a pop-up attached to the button. Since the button's not there you can't use the Ctrl-Click or Shift-Click accelerators on it, but everything else should work.

The Skip next occurrence menu command shouldn't work on messages that were scheduled in other TB profiles.

Improve release notes display:

1. Display the release notes in a Thunderbird tab rather than a separate browser.
2. Display the new and improved release notes page rather than the GitHub releases.
3. If the user's UI locale isn't English, display the Google Translate version of the release notes page.
4. Display the release notes automatically when there's a major or minor upgrade and on initial install, unless the releaseNotesShow advanced preference is set to false.

Fix a bug that was causing the "Send at" button to say, e.g., "every 1 weeks' when it should have been saying "weekly".

The log level wasn't consistently being set properly everywhere from the user's preferences. Now it is.

Eliminate the experiment code for copying new drafts into the Drafts folder and copying messages into the local Outbox folder. Both of these can now be done reliably and successfully using non-experimental APIs. We still have experiment code we can't get rid of yet, but we're gradually reducing the amount of it.

I'm pretty sure there were situations when the "Your Drafts folder is corrupt" message was being shown to the user every minute when that wasn't really the problem. This is has been fixed. Or, at least, some instances of it have been fixed, I can't say for certain if I got all of them.

Translation updates and delete a bunch of obsolete translation strings.