Add a /checks endpoint

To check whether the endpoint wants any files, we can send their SHA256
values:

    POST /checks
    Content-Type: application/json

    { "shas": ["1","2"] }

The endpoint responds with the desired file SHA256 values:

    200
    { "shas": ["2"] }
Show hide key and message forms.
More improvements to UI for signing.
Tweaks.
Add icon font, etc.
More improvements to messaging.
More UI tweaks.
Mobile improvements.
Unfinished work.
These didn't make it.
Tiny shadow.