This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://spring.io/projects/spring-framework with 4 occurrences migrated to:
  https://spring.io/projects/spring-framework ([https](https://spring.io/projects/spring-framework) result 200).

Replace the Google Analytics code with Google Tag Manager.

* pr/1351:
  Remove trailing whitespace
  Fix typo

Closes gh-1351

remove alt attribute in <div>

Polish page.html

Upgrade github-pages gem to fix deprecation warnings

Previously, when running bundle exec jekyll serve a false-positive
deprecation warning was reported:

Deprecation: Auto-regeneration can no longer be set from your
configuration file(s). Use the --[no-]watch/-w command-line option
instead.

See https://github.com/jekyll/jekyll/issues/4831 for some details.
Jekyll 3.0.5 fixes the problem. This commit upgrades to the latest
version of github-pages, thereby also upgrading to Jekyll 3.0.5

Remove default attributes and other little improvements.

Add rouge gem

For copy/paste support in our code samples, we are using the
ZeroClipboard library which relies heavily on Flash.
Because many browsers/extensions are disabling this plugin nowadays,
this commit replaces ZeroClipboard with a native, lightweight solution
for this feature.

The kramdown backend doesn't support titles whithout empty lines before
them.

Update for changes in github pages

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).