Make help button more accessible

Using an `aria-label` for buttons without text is a good practice.

This lets users who use screenreaders better understand what this button
is for.

lib/gollum/locales/en.yml

@@ -1,4 +1,7 @@
 en:
+  editor:
+    function_bar:
+      help: Help
   pagination:
     aria:
       label: Pagination

lib/gollum/templates/editor.mustache

@@ -45,7 +45,15 @@
 			<span class="pr-2"></span>
 			{{/critic_markup}}
       <button class="btn btn-sm function-button" id="function-text-direction" title="Reverse Text Direction">{{#octicon}}arrow-both{{/octicon}}</button>
-			<button class="btn btn-sm function-button" id="function-help" title="Help">{{#octicon}}question{{/octicon}}</button>
+			<button
+				aria-label="{{tt.editor.function_bar.help}}"
+				class="btn btn-sm function-button"
+				data-help-text="{{help_text}}"
+				id="function-help"
+				title="{{tt.editor.function_bar.help}}"
+			>
+				{{#octicon}}question{{/octicon}}
+			</button>
     </div>
 
     <div id="gollum-editor-format-selector">