Allow populating form fields with query parameters for sign/verify/encrypt/decrypt pages

### What is wrong?
- https://keybase.io/sign
- https://keybase.io/verify
- https://keybase.io/decrypt
- https://keybase.io/encrypt

It would be very nice to be able to specify values for the various form fields on these pages using query parameters.
### How could it be fixed?

When a user visits a url like `https://keybase.io/sign?msg=a-messag-to-sign` the page could auto-populate the form field with the message specified in the query parameter. This would make it easy to send a user to this page and have the desired message pre-populated into the textarea.
#### Cute animal picture


Start by opening the sign, verify, decrypt, and encrypt page URLs named in the issue and inspect how their forms handle initial values. Done means supported query parameters pre-populate the relevant fields on each page, including the example message parameter; no source files or tests are identified in the issue.

Written by the indexing model from the issue text.