Ability to sort using form fields (for mobile portrait mode)

We now display sort options as a select box plus a descending checkbox, which
means you can apply sort orders even in portrait mode on a mobile phone where
the column headers are hidden.

Closes #199

datasette/templates/table.html

@@ -66,7 +66,22 @@
                 {% endfor %}
             </select>
         </div><input type="text" name="_filter_value" class="filter-value">
-        <input type="submit" value="{% if filters.has_selections() %}Apply filters{% else %}Add filter{% endif %}">
+    </div>
+    <div class="filter-row">
+        {% if is_sortable %}
+            <div class="select-wrapper">
+                <select name="_sort" id="sort_by">
+                    <option value="">Sort...</option>
+                    {% for column in display_columns %}
+                        {% if column.sortable %}
+                            <option value="{{ column.name }}"{% if column.name == sort or column.name == sort_desc %} selected{% endif %}>Sort by {{ column.name }}</option>
+                        {% endif %}
+                    {% endfor %}
+                </select>
+            </div>
+            <label class="sort_by_desc"><input type="checkbox" name="_sort_by_desc"{% if sort_desc %} checked{% endif %}> descending</label>
+        {% endif %}
+        <input type="submit" value="Apply">
     </div>
 </form>