Widget Visibility: improve compatibility with bbPress' Forums root

Steps to reproduce:
1. Activate Jetpack and the Widget Visibility module
2. Install and activate bbPress
3. Create a new static page named "forumpage"
4. Set that page as Forums root under Settings > Forums
5. Go to Appearance > Widgets, and open the visibility options for one of your widgets:
- Option 1: display widget when page = "Forums" post type => doesn't work.
- Option 2: display widget when page = forumpage => doesn't work

It's currently not possible to apply any Widget conditions to that particular page.

We should see about adding extra rules using [bbPress' conditional tags](http://bbpress.trac.wordpress.org/browser/trunk/includes/common/template-tags.php?rev=4919#L1012) when bbPress is active.

Reported here: [#2474-bbpress](http://bbpress.trac.wordpress.org/ticket/2474) and [#2050-jetpack](https://plugins.trac.wordpress.org/ticket/2050)

Start by reproducing the issue with Jetpack's Widget Visibility module and an active bbPress Forums root, then inspect the linked bbPress conditional tags. Done means widget visibility conditions can target the Forums root and its associated static page when bbPress is active, with the reported steps no longer failing.

Written by the indexing model from the issue text.