topbar base height variable should be adjustable

[imtiazShakil]

the variable $topbar-base-height is used for adjusting the height of .sidebar-brand & .topbar class. But the variable is statically declared without the !default keyword.

This forces user to modify sb-admin2 theme, because they can't override the variable.

So please add a !default keyword for $topbar-base-height variable

[davidtmiller]

I will drop the !default declaration on most of our overrides in the future to make the theme more extendable and adjustable. Thanks for the explanation!