Masthead - add support for taller logos

[kmcfaul]

Per conversation on slack and in rhamilto/console#42 (comment)

MastheadLogo currently sets a max height that restricts child logos, and should support overriding this css variable for taller logos. A minor related issue is that masthead content seems to not be vertically centered (if the toolbar has a full height modifier this isn't as apparent, but then the toolbar items look too tall), so content can look misaligned with a taller logo height.