Search forms need submit button

Search & filter forms need a submit: https://www.w3.org/TR/WCAG20-TECHS/H32.html

eHoldings uses a block-level "Search" button after the <SearchField>...

... but I recommend embedding a solution directly into the <SearchField> component.

There are a few different ways we could approach this:

• Google puts a clickable search icon on the end of the input; most people probably just hit enter, but it's there for people who need it. This option could be pulled off with basically the same style as currently applied, just moving the search icon to the end of the input instead of the beginning:
  
• Here's a nice example with an integrated button with a background color:
  https://designsystem.digital.gov/components/search-bar/

We should also use input type="search" - there are plenty of smart workarounds for styling those now. It unlocks nice things like software keyboards changing "Enter" to "Search" instead of "Submit" and smarter autocompletion.