In an attempt to get MantisBT / MantisTouch redirection to work better o...

In an attempt to get MantisBT / MantisTouch redirection to work better on mantishub.com several issues were fixed.

Fixes #16768: Default mantistouch_url correctly when MantisTouch is installed in 'm' subfolder.
Fixes #16769: MantisTouch redirect can break soap api based on user agent sent.
Fixes #16770: Redirect from MantisBT issue to MantisTouch should go to the same issue page on MantisTouch.