Added applyConfigToAdmin option to manager_config.json

[MartinaeyNL]

Fixes #1287

Added applyConfigToAdmin option to manager_config.json.
It controls if custom page configurations are shown to the admin user.
Defaults to true.

[richturner]

I am not a fan of config that affects the admin user as quickly the admin user cannot perform actions in the manager UI (the admin tool)

[DonWillems]

As someone who edits the manager config using the appearance page, its a bit awkward to do:
"manager": { "applyConfigToAdmin": false }

instead of directly on the same level as 'realms' and 'pages':
"applyConfigToAdmin": false

But if that code-wise makes the most sense, it's ok for me