Conditionalize reference doc/api doc links

If the reference doc URL of the API doc URL is empty, don't display a hyperlink.

This reverts commit bf15ee8a.

This change adds a new "Related Courseware" section in the side
container; this section lists links to courses and talks that are
relevant to the current project page.

Relevant content is fetched by the browser, with an AJAX request sent to
the Pivotal Education LMS API. This API uses the Referrer HTTP header as
input parameter to its content selection algorithm.

Note that CORS HTTP headers are also used to allow/disallow AJAX cross
domain requests.

Gem installation instructions updated

Fix directory name for sample pages

Using "gem install" didn't work for me because of the Gemfile.lock that is in source control (presumably for a reason).

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.