[feature/avatars] Handle deletion of avatars

Previously this wasn't handled correctly if at all.

PHPBB3-10018

phpBB/styles/prosilver/template/ucp_avatar_options.html

@@ -9,6 +9,7 @@
 		<dl>
 			<dt><label>{L_CURRENT_IMAGE}{L_COLON}</label><br /><span>{L_AVATAR_EXPLAIN}</span></dt>
 			<dd><!-- IF AVATAR -->{AVATAR}<!-- ELSE --><img src="{T_THEME_PATH}/images/no_avatar.gif" alt="" /><!-- ENDIF --></dd>
+			<dd><label for="av_delete"><input type="checkbox" name="av_delete" id="av_delete" /> {L_DELETE_AVATAR}</label></dd>
 		</dl>
 	</fieldset>
 	<h3>{L_AVATAR_SELECT}</h3>