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.
Make the link to a project's forum optional

Omit the link to the forum if the project's metadata does not provide
a forum url
Merge pull request #5 from wilkinsona/optional-forum-link

Make the link to a project's forum optional
Merge branch 'gh-pages' of https://github.com/spring-projects/gh-pages into gh-pages
Update links for tutorial and add link to guide