Fix references to message parts in encrypted messages

For a content node in the decrypted mime tree of an encrypted message
content()->index() gives a wrong "relative" index inside the decrypted
mime tree. NodeHelper::fromHREF() which is used to get the content node
for the reference has no idea that is need to look inside the decrypted
message and returns the content node of the encrypted message
corresponding to the reference which is most likely the encrypted message
part. As a result importing an OpenPGP key or a vCard attached to an
encrypted message failed because the wrong message part was passed to
the corresponding URL handlers.
Using (a simpler overload of) NodeHelper::asHREF() (i.e. the counter part
for NodeHelper::fromHREF()) for generating the reference fixes those
problems.

I have verified that this change is safe for all users of
MessagePart::makeLink (which all live in kdepim-addons) and I have tested
that it fixes importing OpenPGP keys and vCards attached to encrypted
messages.

BUG: 469383
FIXED-IN: 5.23.1