github: add zenodo.json compatibility

[alejandromumo]

On legacy Zenodo, we allow users to override any metadata from a release by adding a .zenodo.json.

This was removed from RDM but it can be added by:

• Overriding the release processing metadata, namely by adding the metadata found on .zenodo.json
• Adding a tab "Metadata" to the Github release UI with a helper.

[anikachurilova]

implementation should be something like this:

{%- if release.release_object.status == 'D' %}
    <div class="{{ 'active' if active }} item" data-tab="tab-name3">{{ _('Metadata') }}</div>{% set active = false %}
{%- endif %}

{%- if release.release_object.status == 'D' %}
  <div class="ui tab {{ 'active' if active }}" data-tab="tab-name3">
    <div class="ui grid">
      <div class="row">
        <div class="column">
          <h4>{{_('JSON Export')}}</h4>
        </div>
      </div>
      <div class="row">
        <div class="column">
          <p>
            {{config.THEME_SITENAME | default('Invenio')}} automatically
            extracts metadata about your repository from GitHub APIs.
            For example, the authors are determined from the
            repository's contributor statistics.
            The automatic extraction is <strong>solely a best
            guess</strong>. Add a
            <code>{{config.GITHUB_METADATA_FILE}}</code> file the root of
            your repository to explicit define the metadata. The format of
            file is the same as for our REST API (use e.g. below JSON to
            get started).
          </p>
        </div>
      </div>
      <div class="row">
        <div class="column">
          <pre>{{ release }}</pre>
        </div>
      </div>
    </div>
  </div>
  {% set active = false %}
{%- endif %}

[alejandromumo]

UI not yet implemented, maybe we can add it later on. For now having the backend parsing the CITATION.cff and .zenodo.json should be a priority