Sign the index.jar file using Kerplapp keypair/signing cert.

No longer using the testkey mode of zipsigner-lib to sign the index.jar.
Signature is now applied using the keypair & cert associated with the
INDEX_CERT_ALIAS in the keystore.

Without zipsigner-lib-optional on the classpath zipsigner generates
ClassNotFound exceptions when using custom keys. Unfortunately this means yet
another jar in the libs directory :-/
Generate an index.html to bootstrap the repo.

Implements Feature #2668 : index.html for bootstrap.

When the IP address is set from wifi the KerplappRepo now generates an
index.html in the wwwroot of the repo. If the Kerplapp host has the F-droid
client APK installed it will be symlinked (or copied) into the wwwroot and used
by the bootstrap page for clients to install F-droid. In the event you have just
Kerplapp and not F-droid the link points to the internet accessible F-droid APK.