Add bulk actions to the Stores admin

* Add brand.
* Set Chain.

app/stores/templates/admin/add_brand.html

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+{% extends "admin/base_site.html" %}
+
+{% block content %}
+
+<p>Select brand to apply:</p>
+
+<form action="" method="post">
+
+    {{ brand_form }}
+
+    <p>The brand will be applied to:</p>
+
+    <ul>{{ stores|unordered_list }}</ul>
+
+    <input type="hidden" name="action" value="add_brand" />
+    <input type="submit" name="apply" value="Apply tag" />
+    {% csrf_token %}
+</form>
+{% endblock %}

app/stores/templates/admin/set_chain.html

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+{% extends "admin/base_site.html" %}
+
+{% block content %}
+
+<p>Set the store's chain to:</p>
+
+<form action="" method="post">
+
+    {{ chain_form }}
+
+    <p>The following stores will be set:</p>
+
+    <ul>{{ stores|unordered_list }}</ul>
+
+    <input type="hidden" name="action" value="set_chain" />
+    <input type="submit" name="apply" value="Apply tag" />
+    {% csrf_token %}
+</form>
+{% endblock %}