[BUGFIX] Prevent invalid VariablesViewHelper rendering

This patch resolves two problems when rendering powermail_rte contents.

The parseFunc wraps {powermail_all} in a paragraph tag. To prevent
invalid HTML the paragraph is removed using preg_replace.

Additinally the html_entity_decode() call will return UTF-8 encoded
data. This prevents problems in some PHP installations where the
$nbsp; character is encoded to an invalid character.

Relates: ##61118
Releases: 2.1
Change-Id: I1cba0b302b87f55b00033cc20f00ecfadb431ec3