Allow organizations to define a custom Welcome text

This allows organizations to add links to their own First Start guide
(or similar documentation) to the Welcome page shown to users with an
empty keyring. The config entry which defines the custom Welcome text
must be marked as immutable.

(cherry picked from commit 4f321f37ac102f925848c01c0e50f37043fe0d50)

• GnuPG-bug-id: T7967