Update web page URL to GitHub website

<?xml version="1.0" encoding="utf-8"?>

<resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">

    <string name="app_revision_url">https://github.com/k9mail/k-9/wiki/ReleaseNotes</string>

    <string name="app_webpage_url">http://code.google.com/p/k9mail/</string>

    <string name="app_webpage_url">https://k9mail.github.io/</string>

    <string name="message_header_mua">K-9 Mail for Android</string>

    <!-- Licenses - translating legal stuff should be left to experts -->