Add new icons for sign / encrypt

* src/icons/Makefile.am (EXTRA_DIST): Update accordingly.
* encrypt-20.png, encrypt.svg, sign-20.png, sign-enc-40.png,
sign-enc.svg, sign.svg: New.

This is more in the visual language of Outlook with the ribbon
as a signature icon. It's also using the new lock of gpgol.