Show paid status in admin and filter CSV exports by year (#30)

## Summary
Closes #30.
- Green check on membership/person admin when a current-year payment email matches
- Year picker on membership and payment CSV downloads
- Payments admin search by email, plus year list filters

schasite/templates/admin/csv_year_export.html

@@ -0,0 +1,29 @@
+{% extends "admin/base_site.html" %}
+{% load i18n %}
+
+{% block content %}
+<form action="" method="post">{% csrf_token %}
+  <p>{{ help_text }}</p>
+  <fieldset class="module aligned">
+    <div class="form-row">
+      <label for="id_year">Year:</label>
+      <select name="year" id="id_year">
+        <option value="all">All years</option>
+        {% for year in years %}
+        <option value="{{ year }}"{% if year == default_year %} selected{% endif %}>{{ year }}</option>
+        {% endfor %}
+      </select>
+    </div>
+  </fieldset>
+  {% for selected_id in selected_ids %}
+  <input type="hidden" name="_selected_action" value="{{ selected_id }}">
+  {% endfor %}
+  <input type="hidden" name="action" value="{{ action_name }}">
+  <input type="hidden" name="select_across" value="{{ select_across }}">
+  <input type="hidden" name="index" value="{{ index }}">
+  <div class="submit-row">
+    <input type="submit" name="apply" class="default" value="Download CSV">
+    <a href="{{ changelist_url }}" class="button cancel-link">{% translate "Cancel" %}</a>
+  </div>
+</form>
+{% endblock %}