Add brand autocomplete dropdown to battery add and device install forms

Uses HTML datalist populated from distinct brands in the battery table.
Removed autocomplete="off" which suppresses datalist in Chrome/Chromium.

templates/battery_add.html

@@ -9,7 +9,10 @@
     <div class="form-group">
       <label for="brand">Brand <span class="text-danger">*</span></label>
       <input type="text" id="brand" name="brand" value="{{ form_brand|default('') }}"
-             placeholder="e.g. Panasonic Eneloop" required>
+             placeholder="e.g. Panasonic Eneloop" list="brand-list" required>
+      <datalist id="brand-list">
+        {% for b in brands|default([]) %}<option value="{{ b }}">{% endfor %}
+      </datalist>
     </div>
 
     <div class="form-group">