[Feature][0.9][Merged] Custom admin middleware

[tabacitu]

Builds upon #67 . I'm going to copy-paste my last comment from there, which explains everything this PR does.

Update: I still agree admin was an uninspired name for this middleware.

But after implementing the guard system in PR #165, I think a more future-proof solution would be to allow the developer to use a custom name, if he so chooses. Or a different middleware, while we're at it. We can fetch the middleware using helpers, and those helpers could get the value from the config. Expanding on how @OwenMelbz did it in PR #87 .

To rephrase, in config/backpack/base.php we'd have:

    // Fully qualified namespace of the User model
    'user_model_fqn' => '\Backpack\Base\app\Models\BackpackUser',

+    // The class for the middleware that checks if the visitor is an admin
+    'middleware_class' => \Backpack\Base\app\Middleware\Admin::class,
+
+    // Alias for that middleware
+    'middleware_key' => 'admin',
+    // Note: It's recommended to use the backpack_middleware() helper everywhere, which pulls this key for you.

This way:

• the developer could change the admin key for the middleware to anything he wants;
• the developer could easily use a different middleware, if he so chooses; just create a custom class, then point to that class in the config;
• the developer could easily use the middleware name, thanks to the helper; so it's backpack_middleware() instead of config('backpack.base.middleware_key'); easiler to remember; easier on the eyes;

Other considerations:

• keeping the admin name for the middleware will make this backwards-compatible; a non-breaking change; but we could force the name change to checkIfAdmin to make it more Laravely and make sure people upgrade their Backpack correctly; food for thought;
• the current admin middleware is very basic; it just checks for authentication and makes sure the redirect uses the route_prefix; that's the only reason we didn't use the auth middleware; so I expect anyone who has both users & admins to change this middleware to something else; wether they use PermissionManager or not;
• we'll still need to instruct people to use this helper in their own Controllers or route files; this can be in the upgrade guide;

Thoughts? @ChrisThompsonTLDR , @OwenMelbz , @ctf0 , @eduardoarandah , @lloy0076 you guys seemed to be interested in this feature. Let me know if you think there's a better solution.

[tabacitu]

Merged into upgrade branch #259 . We'll leave this PR and branch open, and push fixes here if anything comes up in testing.