Add subtitle to site masthead.

[tpendragon]

Closes #573

[hackartisan]

Having the subtitle makes the masthead taller so that going to an exhibit with no subtitle puts the search bar in a different place and feels like an awkward transition. Could we pad when the subtitle is missing?

[tpendragon]

@HackMasterA If I did that it would look like this:

That's preferrable?

[hackartisan]

Yeah it's not great. What if we pull the title down when there's no subtitle? Then the title would jump around a bit but I think that would be less disruptive than the whole masthead changing size. Or maybe you have some other idea/s. Should we user test a few things? Ask for more opinions?

[escowles]

What if we kept a min-height to keep the header the same height, but reduced the space between the subtitle and the search bar? That seems like a reasonable way to tradeoff having the title stay in one place, but not have a ton of extra space when there's no subtitle.

[coveralls]

Coverage remained the same at 100.0% when pulling 55f3313 on masthead_subtitle into ad54bc4 on master.