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.

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 reverts commit c2665868.

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.

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.

Links added:

* part of the reference documentation listing mave/gradle coordinates
* screencast published on the Spring blog (creating and deploying a REST
app under 5 minutes)

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.

Since the main example here is using the `spring-context` module,
Spring developers should be aware of all the other existing Spring
modules and their artifacts names.

A link to the corresponding section in the reference documentation is
added.

Issue: SPR-11534

Use constructor injection

Conflicts:
	_includes/billboard.html

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.