BUG Consistently self-closing form field tags (#7557)

This is reverting templates back to pre-3.0 conventions,
which were dominated by XHTML templates.
HTML5 allows both self-closing and unclosed tags,
so opting for self-closing to achieve maximum compatibility.

templates/forms/CheckboxField.ss

@@ -1 +1 @@
-<input $AttributesHTML>
+<input $AttributesHTML />

templates/forms/CheckboxSetField.ss

@@ -2,7 +2,7 @@
 	<% if Options.Count %>
 		<% loop Options %>
 			<li class="$Class">
-				<input id="$ID" class="checkbox" name="$Name" type="checkbox" value="$Value"<% if isChecked %> checked="checked"<% end_if %><% if isDisabled %> disabled="disabled"<% end_if %>>
+				<input id="$ID" class="checkbox" name="$Name" type="checkbox" value="$Value"<% if isChecked %> checked="checked"<% end_if %><% if isDisabled %> disabled="disabled"<% end_if %> />
 				<label for="$ID">$Title</label>
 			</li> 
 		<% end_loop %>

templates/forms/FileField.ss

@@ -1,2 +1,2 @@
-<input $AttributesHTML>
-<input type="hidden" name="MAX_FILE_SIZE" value="$MaxFileSize">
+<input $AttributesHTML />
+<input type="hidden" name="MAX_FILE_SIZE" value="$MaxFileSize" />

templates/forms/FormAction.ss

@@ -3,5 +3,5 @@
 		<% if ButtonContent %>$ButtonContent<% else %>$Title<% end_if %>
 	</button>
 <% else %>
-	<input $AttributesHTML>
+	<input $AttributesHTML />
 <% end_if %>

templates/forms/FormField.ss

@@ -5,5 +5,5 @@
 		$Value
 	</span>
 <% else %>
-	<input $AttributesHTML>
+	<input $AttributesHTML />
 <% end_if %>

templates/forms/OptionsetField.ss

@@ -1,7 +1,7 @@
 <ul id="$ID" class="$extraClass"<% if Description %> title="$Description"<% end_if %>>
 	<% loop Options %>
 		<li class="$Class">
-			<input id="$ID" class="radio" name="$Name" type="radio" value="$Value"<% if isChecked %> checked<% end_if %><% if isDisabled %> disabled<% end_if %>>
+			<input id="$ID" class="radio" name="$Name" type="radio" value="$Value"<% if isChecked %> checked<% end_if %><% if isDisabled %> disabled<% end_if %> />
 			<label for="$ID">$Title</label>
 		</li>
 	<% end_loop %>