support context path for admin servlet

[thoniTUB]

Currently the admin servlet does not really support a configuration like this:

    "server": {
        "adminContextPath": "/test",
        "applicationContextPath": "/test"
    }

where the wohle servlet is mounted under a subpath. (/admin-ui becomes /test/admin-ui) The result is a broken UI and other links.

However it would be very useful for hosting multiple instances (testing) under a single domain.

Support for the application servlet comes in another PR.