[join] PIN input box UX

[yoursunny]

The ndncert PIN input should automatically strip whitespace.
When I double click the PIN in email and copy it, a whitespace can be included easily.
Since it's known that the testbed CA would not include a whitespace in the PIN, stripping the whitespace could improve user experience.

Since it's known that the testbed CA would only use digits in the PIN, the PIN input field should open with numeric keypad instead of alphabetical keyboard on a mobile browser.