Add support for changing the browser redirection timeout

[xmission]

Rather than a static 3 minutes, allow users to set a custom redirection timeout.

The timeout could also be set dynamically based on the size of the plaintext. Current reading speeds are about 250 WPM with 70% comprehension average.

The timeout could be encrypted with the plaintext, and when decrypted, sent to the template, or could be added as a URL addition with something like "?t=300" for 300 seconds.

[atoponce]

The URL addition could work, as the sender would be able to set the time as he sees fit in the URL without form submissions. When the recipient receives the URL, they must change the time before sending the GET request, otherwise the note is destroyed, and refreshing the URL with a different timeout would fail.