Add close/cancel button to all modals

[stsewd]

The only modal that I didn't put a button on is

ext-theme/readthedocsext/theme/templates/organizations/partials/owners_list.html

Lines 24 to 36 in f45f878

	<div class="ui mini modal" data-modal-id="invite-owner">
	<div class="header">{% trans "Invite organization owner" %}</div>
	<div class="content">
	<form class="ui form"
	method="post"
	action="{% url "organization_owner_add" organization.slug %}">
	{% csrf_token %}
	{{ form|crispy }}
	<input class="ui button" type="submit" value="{% trans "Invite owner" %}">
	</form>
	</div>
	</div>
	{% endif %}

since it has the form as content, doesn't have an "actions" div.

Closes #510