Fix "rm -rf" instruictions

Existing copy is wrong because "git rm -rf *" fails if there are any untracked files (e.g. target/). Changed to "git rm -rf `git ls-files`".

Update navbar item "Forum" -> "Questions"

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

Fix ZeroClipboard integration in gh-pages

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

Flash support for ZeroClipboard is not working in project pages since
the SWF file is missing. There was a typo in the .gitignore file that
ignored "*.sw?" files.

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.