doc: Add security warnings to README; fix wording

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
README: Fix mailto: link

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: Bump stoken to rev 4637c7489

This picks up the new class GUID API and formatTokencode() API.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
ImportActivity: Rework handling of bound device IDs

Try guessing our computed ID and the standard class GUIDs first; if
that succeeds, autofill the form.  If that fails, give the user a better
indication of whether the devid or the password is incorrect.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Misc: readStringFromUri: Cap the input length

Selecting a large file can result in an OutOfMemoryError.  Limit the
input to 64kB to prevent this.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>