When served with HTTP/2 and TLS, project pages show several mixed
content errors:
* the imported Google fonts
* the AJAX call to spring.io to get the current project information
* the newsletter iframe
This commit changes URLs to use https or protocol-relative URLs when
possible. This also deletes the now defunct newsletter integration.
Remove the use if "unless contains 'not found in _includes directory'"
hacks that were previously used to detect when _includes were missing.
This change is required as the hack no longer works with the latest
Jekyll release (and therefore also with GitHub).
Additional 'custom_pom_template' and 'custom_gradle_template' flags
must now be set in _config.yml if you want to include a custom pom.xml
or build.gradle file.
