Customize presentation of accept/decline buttons in iMip mail

Fix Issue #11230
Only present accept/decline button links in iMip mail for REQUEST, not CANCEL or others.

Fix Issue #12156
Implement config setting "dav.invitation_link_recipients", to control
which invitation recipients see accept/decline button links.  The
default, for public internet facing servers, is to always include
them.  For a server on a private intranet, this setting can be set
to the email addresses or email domains of users whose browsers can
access the nextcloud server referenced by those accept/decline
button links. It can also be set to "false" to exclude the links
from all requests.

Signed-off-by: Brad Rubenstein <brad@wbr.tech>