Add dataset feed to homepage

[luistoptal]

Pull request for issue: #199

IMPORTANT: this is a follow up to PR #2078, I suggest reviewing #2078 and this PR in combination as this one includes refactor to the changes in #2078

• Adds a dataset feed to the homepage
• Refactors the news carousel as a resuable shared partial

How to test?

In home page http://gigadb.gigasciencejournal.com/ you should see a responsive carousel with datasets

How have functionalities been implemented?

• the news feed from Homepage news items section rework #2078 was refactored to convert the carousel into a reusable partial protected/views/shared/_carousel_slider.php
• In the SiteController a dataset search is performed and the search result passed to the view to be able to render a list of datasets as a carousel
• The number of datasets on display is limited to 12, but more can be fetched
• If more than 0 datasets are returned then render the dataset feed section protected/views/site/datasets_carousel.php

Any issues with implementation?

• if the user changes the current carousel slide and then resizes the window, the carousel position is reset to slide 1. I think this is a fringe usecase so I did not handle it
• The dropdown to select type from the original example is not implemented, I am not sure if it is useful. If this is an important feature, it could be added
• Regarding the dataset data, I see a handful of utility methods to 'get' dataset data, e.g. protected/components/FormattedDatasetMainSection.php, in this PR I am directly accessing the dataset values but I do not know if I should use these utilities instead, for example, to get the dataset title i do:

$dataset['title']

But I think that it might be possible to alternatively do something like:

(...)
$mainSection = $assembly->getDatasetMainSection();
(...)
$mainSection->getHeadline()['title'];

Any changes to automated tests?

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