The replay interface and text field still lack clarity

[machawk1]

In #406 we improved the ipwb replay UI to have a more suggestive placeholder value and a submit button for accessibility:

...however, it is still unclear in this interface what it means to enter a URI and hit submit. What is it submitting and searching? What is "the archive"? Why am I entering a URI/URL and not free text, as is usually performed with large prominent text boxes?

Should we add a "help" or "more info"? Can we expect users to know what a "memento" is?