box_token_manager: restore admin settings form to initialize/regenerate the Box system token

Problem/Motivation

The box_token_manager submodule has no administrative UI for
managing the Box system OAuth2 token. The only way to (re)initialize the token
is the /boxcom/validate callback, which is the return leg of the
authorize flow — there is no page that starts that flow.

As a result, an administrator cannot initialize or regenerate the Box system
token from the UI. Earlier releases exposed a settings page with a single button
that kicked off the box.com authorize flow; that page is not present in the
current release, so bootstrapping (or rotating) the system token now requires
manual/out-of-band steps.

Steps to reproduce

1. Install oauth2_token_manager with the
   box_token_manager submodule and configure the Box OAuth2
   client credentials.
2. As an administrator, look for a page to initialize or regenerate the Box
   system token.
3. Observe there is no such page under Configuration » Web services (or anywhere else).

Proposed resolution

Add an admin settings page that starts the authorize flow.

User interface changes

Adds a new admin page at
/admin/config/services/box_token_manager ("Box OAuth2
Authentication") with a single button to initialize / regenerate the Box system
token, plus a menu link under Configuration » Web services.
Visible only to users with the administer box token manager
permission.

API changes

None. A new form class, route, and menu link are added; no existing APIs
change.

Data model changes

None.