[BUGFIX] Use custom parseFunc in email templates

Since the content of the FlexForm RTE fields is now processed
by the ts_css transformation the contents need to be parsed by
lib.parseFunc_RTE so that the final HTML in the mails is correct.
Additionally the links need to be absolute.

This patch introduces a new parseFunc (lib.parseFunc_powermail)
that is used by the email templates. It is based on
lib.parseFunc_RTE and forces absolute link generation.

Resolves: #61118
Releases: 2.1
Change-Id: Ib06bee01c793a36be64cb18a3926cc947a57ad49