ImagePage: Do not parse message 'imgmultipageprev'

ImagePage: Do not parse message 'imgmultipageprev'

This message is used as alt text for an image thumbnail, and so it
can't contain HTML tags anyway. This matches the 'imgmultipagenext'
message now. (This inconsistency has been present ever since the
feature was introduced in 2006: 7f7e79022bfd421e49084b089e74c7a16b77bc03.)

Due to using OutputPage::parse() instead of Message::parse(), an
incorrect `<div class="mw-parser-output">` wrapper was also being added.

Bug: T166508
Change-Id: I092887cc91e2c800685d4e55177aed814bb3fce9