On search results page, digital filter checkbox reverts search term

[HaSistrunk]

Is your feature request related to a problem? Please describe.

As observed in usability testing, selecting the digital matches filter checkbox before submitting a search did not follow user expectations, resulting in the filtering of the results from their previous search.

Steps to reproduce

1. Submit a search for any term
2. Now enter a new term, but do not submit the search. Instead, select the "show only results with digital matches" checkbox
3. Notice that the search term reverts to the previous term and applies the filter to that term.

Describe the solution you'd like

I initially thought this was a bug, but in articulating the issue I realize that this is both about pattern consistency and understanding the action of the checkbox. Selecting it before submitting a search term is required from a homepage search, but it functions differently (as a post-search filter) on the search results page.

I have two ideas:

1. Do nothing. Error recovery was not a challenge as observed in testing, and the pattern is easy to learn even if it doesn't follow initial expectations. Problem was observed in one test, so track in future testing.
2. Change the checkbox functionality so that it is tied to the search submit button. I'm not sure how easy this is, but it also has the downside of removing the toggle functionality of the checkbox.

Additional context

See PR #374.