Change "admin_mode_path" from a GET to a POST

[nimmolo]

#2048 - Prevent accidentally turning on Admin mode by hovering over the Admin button.

Changes the action from show to create in Admin::SessionController.

[coveralls]

coverage: 94.418%. remained the same
when pulling 945ff02 on 2048-POST-admin-mode
into 723ed91 on main.

[JoeCohen]

@nimmolo: Thanks for doing this.

1. It does work to prevent accidentally entering/leaving admin mode, which is great.
2. When explicitly entering/leaving admin mode, the page you end up on is sort of random.
   It appears to depend on where your cursor has been. Maybe it's what was last eager loaded.
   Perhaps deal with that in a separate PR?

[nimmolo]

That's true, huh. I imagine it is the last eager loaded page you end up on, and I can't really imagine how we could override that... this preloading is a new twist.