Clean up `bp_blogs_add_user_to_blog()`:

* Compare user roles against `get_editable_roles()` so dynamic roles cannot be accidentally used.
* Use `array_shift()` rather than `array_search()` to speed up comparison.
* Add brackets and inline doc.

See #5749.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8598 cdf35c40-ae34-48e0-9cc9-0c9da1808c22