Create new role for handbook editors

[samuelsidler]

We should create a role specifically for people who edit the handbooks to segment them from other content on devhub. The role would only allow editing of pages in the handbooks, but not creation of new pages.

It'd also be great if we could auto-expire the role. That is, if a user with the "handbook editor" role was inactive for x months, we'd convert them to a "subscriber" role. If we do that, we'll want to tell them that the role will expire in the registration email or perhaps tell them the next time they try to login to developer.WordPress.org. Perhaps something like "Your editor user level has expired due to inactivity. Please contact XXXX to have it renewed."

[coffee2code]

In 930:

Code Reference: add new role for handbook editors. Fixes #621

[coffee2code]

There is now a "Handbook Editor" role. That role is solely geared towards editing just the handbooks. Differing from the original request, they *can* create new handbook pages, but cannot publish them. This seems reasonable and is easier to implement. They can upload media as well (which they probably need to do in the course of editing a handbook page -- namely to add images).

I also restricted handbook access to just administrators, editors, and handbook editors.

The auto-expire aspect was not implemented and should be requested in a separate ticket if still desired. One thing to clarify in that ticket would be what is considered inactivity (editing handbook pages, visiting the backend of the site)? Will handbook pages go through enough regular updating to keep everyone "active"?