Fix regressions with adopters

The fetching of adopters for individual projects no longer works. And the plugin api was removed in favour of data-attributes. However this plugin api was still being used by various project sites:

<script>
    eclipseFdnAdopters.getList({
        project_id: "iot.hono",
        selector: ".eclipsefdn-adopters",
        logo_white: true
    });
</script>

vs.

<div class="eclipsefdn-adopters" data-project-id="iot.hono" data-logo-white="true"></div>

The solution for this would be to re-add the plugin API and re-implement the project adopters.

As a side note, the documentation pages need to be updated on the adopters api repo and EFSA's documentation website.