iMIP email improvements (take 2)

This PR is a replacement for PR #17195. It is intended to be simpler
to review and approve, with fewer changes, some disabled by default.

It addresses issues #12391 and #13555, with the following changes:

- The plainText of iMIP emails has been upgraded as described in
issue #12391. The HTML design style has not been changed.

- Some of the HTML and plainText content has been rearranged
(simplified header language, moving the event title to from text
body to the first item in the bullet list, spelling corrections,
moving the description to the end of the list), per issue #12391.

- The interface for EMailTemplate has been extended: addBodyListItem
now takes an optional `plainIndent` parameter. Existing callers
see no change. Where new calls set the  new parameter >0, the list
item label (metaInfo) is put in column 1, and the value is indented
into column 2 (properly accounting for multiple lines, if any).

- An optional dav config setting has been added,
`invitation_list_attendees`. It defaults to 'no', leaving emails
unchanged. If set by the site admin to 'yes', then iMIP emails
include, for the organizer and each attendee, their name, email,
and a ✔︎ if they have accepted the invitation.

- Minor refactoring.

Notes:

- The labels for organizers and attendees list items are new, and
require translation/localization.

- Dav config settings are documented in the code, but not in the
Administrator's Guide.

Signed-off-by: brad2014 <brad2014@users.noreply.github.com>

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