Ability to restrict plugins and themes to one journal

[NateWr]

Describe the problem you would like to solve
In a large, multi-tenant instance of OJS, each journal may want its own custom theme. However, when a theme is installed in the site it becomes visible and available to all journals in the site.

Describe the solution you'd like
It should be possible to restrict plugins and themes to one or more journals. This could be done simply by hiding a plugin when it exists on the filesystem but isn't yet "installed" by a journal. Or it could be possible to install distinct copies of a plugin for each journal.

Who is asking for this feature?
PKP|PS. The primary use-case is for handling small theme customizations for lots of journals. A small theme plugin might only override one or two templates, but with hundreds of journals in a system, it can lead to a very large number of installed themes.

Ideally, a theme installed by a journal would only be available to that journal unless the site administrator wanted it to be available everywhere.

Additional Comments
cc @mfelczak