Move dav.invitation_link_recipients from getSystemValue to getAppValue

Per @georgehrke change request for PR #12392, instead of setting
dav.invitation_link_recipients in the system config.php file, we
set it in the database table oc_appconfig.

Furthermore, the value of the config variable is always a string:
'yes' to include links in imip mail, 'no' to exclude them, or a
comma-separated list of email addresses and/or domains for which
they should be included.  If not specified in oc_appconfig, the
default is 'yes'.

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