Can't set brief view level to none

[kwindrem]

There is no way to select None for any of the levels. There are systems that don't have that many tank sensors and/or you do not want to display something on the Brief page but do want it on the tanks page.

[chriadam]

I think it's something which can be supported in future release.
So, not for 1.0.0.
Marking as low_prio for now.

[blammit]

User needs to be able to choose the centre gauges that are shown on the Brief page.

See also #1491