Add actions before & after register template part fieldset fields. Fixes #2534.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5303 9866e705-20ec-0310-96e7-cbb4277adcfb
Use correct filter name in `bbp_get_forum_slug()`. Props netweb. Fixes #2525.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5304 9866e705-20ec-0310-96e7-cbb4277adcfb
Use `height: auto;` alongside `max-width: 100%;` in `bbpress.css` to maintain image aspect ratio. Props robin-w. Fixes #2553

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5306 9866e705-20ec-0310-96e7-cbb4277adcfb