Various gh pages maintenance changes

See https://github.com/spring-projects/spring-xd/pull/1197

Remove 'not found in _includes' hacks

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.

Change Forum navbar item to Questions

The projects navbar should mirror the one on the spring.io website.

Fix company name and website in footer