Update highlightlng for maven and gradle snippets

Replace the highlighting logic used for maven and gradle snippets with
Jekyll code snippet highlighting (`{% highlight %}`). This change both
simplifies `widget_templates.html` and allows external pom/gradle
includes to be highlighted.

The change also splits `widget-template` into two distinct elements,
one for the dependencies and one for repositories. This is necessary
due to the fact the the `{@ @}` condition statements used by
`underscore.js` are incorrectly highlighted as errors when parsing
`gradle` snippets (preventing them from being recognized). Splitting the
repository template has the additional benefit of custom pom/gradle
files no longer need to specify a repository section.

[Finishes: #55617424]
Merge pull request #2 from philwebb/55617424

* 55617424:
  Update highlightlng for maven and gradle snippets