S2: add viewer_is_maintainer()

[ex_uniquewo]

Title
S2: add viewer_is_maintainer()

Short, concise description of the idea
I'd like to have some elements of the sidebar only visible to my co-maintainers in order to create an admin tools box.

Full description of the idea
viewer_is_owner() lets you have private elements in one's paid journal but it doesn't work in communities. Either make it work for these too or create viewer_is_maintainer() for communities.
An ordered list of benefits

• Let community maintainers have a hidden handy link list to easily admin their paid communities.

An ordered list of problems/issues involved

• None?