Add an option to enable mobile site, but not redirect mobile users

[sminnee]

From http://open.silverstripe.org/ticket/5952

It should be possible to enable the mobile site to mobile users, or anyone by directly accessing the URL, e.g. "m.mysite.com" without any redirection or automatic mobile theme switching if a mobile user accesses mysite.com. This would be more of an "opt-out" approach, so the mobile site exists, but the full site has a link advertising the fact there's a mobile site available.

This would give us "Mobile site behaviour" options like so:

Disabled (default)
Enabled
Enabled, redirect mobile devices
Enabled, show mobile theme to mobile devices without redirection
It might also be helpful to include help text for each of these options, to explain the behaviour.