Add setting to admin pane for enabling comments

This change set adds an "enabled/disabled" setting to the BP-Docs admin pane for enabling comments on BP Docs. 

![bp_docs_setting](https://cloud.githubusercontent.com/assets/1391994/4980079/c0e615ac-68fc-11e4-97c5-bd0507edfb2d.png)

It also uses the new stored setting to determine whether or not to add read/leave comments access settings to the access pane on the doc create/edit page. (So users won't have to set a comment setting if comments are not enabled globally.)

![doc_access_panel](https://cloud.githubusercontent.com/assets/1391994/4980090/d2b5199a-68fc-11e4-8e6f-a20d597dc12a.png)
![doc_access_summary](https://cloud.githubusercontent.com/assets/1391994/4980088/ce95d606-68fc-11e4-80e1-dae21287a9d5.png)

The changes that matter are in 610d839c0ef22c55bb8f9e926a71212c9bc26d38 and 6e2cafd406769d6fc81fc5a09af7d2da4dee1778 .

Also note that these changes don't cause any problems in the branch https://github.com/dcavins/buddypress-docs/tree/access-option-defaults