Enhancement: Independent session management for Admin panel

[Sommerregen]

Dear Grav Team,

as already mentioned in the gitter forum I want an indepedent session/cookie managment option for the admin panel. The use case is: As long as a visitor browsing through the site, I don't want Grav to set cookies/session. Only when a special route like /admin Grav should enable session/cookies in order to do a proper login (without you will always be redirected to the login page, which is clear why ;-)).

What I mean is using a different option than the sessions system option or at least to ignore the global flag session.enabled and assume that a session always have to be enabled for the admin panel.

[Sommerregen]

As an addition it would be good to set a different session.lifetime value for admin sessions, too.