Fix duplicated rel attribute (#2549)

According to the HTML 5.1 recommendation "The rel attribute on a
and area elements controls what kinds of links the elements create.
The attribute’s value must be a set of space-separated tokens."
(source https://www.w3.org/TR/html51/links.html#links-created-by-a-and-area-elements)

templates/base/head.tmpl

@@ -210,7 +210,7 @@
 													<i class="octicon octicon-settings"></i>
 													{{.i18n.Tr "your_settings"}}<!-- Your settings -->
 												</a>
-												<a class="item" target="_blank" rel="noopener" href="https://docs.gitea.io" rel="noreferrer">
+												<a class="item" target="_blank" rel="noopener noreferrer" href="https://docs.gitea.io">
 													<i class="octicon octicon-question"></i>
 													{{.i18n.Tr "help"}}<!-- Help -->
 												</a>
@@ -234,7 +234,7 @@
 
 								{{else}}
 
-									<a class="item" target="_blank" rel="noopener" href="https://docs.gitea.io" rel="noreferrer">{{.i18n.Tr "help"}}</a>
+									<a class="item" target="_blank" rel="noopener noreferrer" href="https://docs.gitea.io">{{.i18n.Tr "help"}}</a>
 									<div class="right menu">
 										{{if .ShowRegistrationButton}}
 											<a class="item{{if .PageIsSignUp}} active{{end}}" href="{{AppSubUrl}}/user/sign_up">