Clean up `BP_Blogs_Template()` methods:

* Add inline doc
* Use `public` modifiers
* Normalize comparisons
* Add default variable values
* Add brackets and bring code formatting up to date
* Use `empty()` checks where appropriate

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8758 cdf35c40-ae34-48e0-9cc9-0c9da1808c22