Add support for multipart/digest messages

Expected behavior

In the attached message, which is multipart/digest, there are two parts:

1. An e-mail (message/rfc822), containing a multipart/mixed message which announces an event. It contains an image and a pdf file, as well as text/plain and text/html parts.
2. Another e-email, containing a calendar item for the event. This is another multipart/mixed message, containing a calendar item (text/calendar and application/ics, as well as text/plain and text/html parts).

Thunderbird is capable of uaably displaying the message.
K-9 should too.

Both e-mails should be displayed, including their in-line images.

It should be possible to put the event on the calendar - the event is provided in two machine-readable formats, at least one of which should work. text/calendar and application/ics. There are also text/plain and text/html renditions. The browser should not be involved - even if the item can not be put on the calendar, it should be possible to view the HTML version and/or save the .ics file.

Actual behavior

K-9 does not display the (inline) images in the first e-mail. The attached document link is not displayed, although the attachment can be opened.

K-9 shows the second e-mail as an attachment, which opens its text/plain part the in the web browser, not as an e-mail. The calendar item is not accessible.

Steps to reproduce

1. Introduce the attached file into an imap folder
2. Open it with k-9
3. For comparison, open the file with Thunderbird./Lightning

Note: The sample message is dated 15-Mar-19, 11:35
It is an actual message, with a few substitutions to avoid posting personal information. Thunderbird displays the attached (modified) version, but the DKIM signature is broken as a result.

The message is contained in this ZIP file

Environment

K-9 Mail version: 5.600

Android version: 9 PPWS29.69-26-5 (Security patch 1-Feb-2019) on Moto X-4

Account type (IMAP, POP3, WebDAV/Exchange): IMAP

Start by downloading the linked Sample-Digest.zip and reproducing the message in K-9 Mail, comparing its handling with Thunderbird. Trace how nested multipart messages, inline images, attachments, and calendar parts are displayed; done means both embedded e-mails and their contents are accessible without opening them in the browser, including viewing or saving the calendar item.