Fix link/origin referrer and login redirect (#36279)

Fix #35998

1. Fix `<a rel>` :
    * "_blank" already means "noopener"
* "noreferrer" is already provided by page's `<meta name="referrer">`
2. Fix "redirect_to" mechisam
* Use "referer" header to determine the redirect link for a successful
login
3. Simplify code and merge duplicate logic

templates/base/footer_content.tmpl

@@ -1,7 +1,7 @@
 <footer class="page-footer" role="group" aria-label="{{ctx.Locale.Tr "aria.footer"}}">
 	<div class="left-links" role="contentinfo" aria-label="{{ctx.Locale.Tr "aria.footer.software"}}">
 		{{if ShowFooterPoweredBy}}
-			<a target="_blank" rel="noopener noreferrer" href="https://about.gitea.com">{{ctx.Locale.Tr "powered_by" "Gitea"}}</a>
+			<a target="_blank" href="https://about.gitea.com">{{ctx.Locale.Tr "powered_by" "Gitea"}}</a>
 		{{end}}
 		{{if (or .ShowFooterVersion .PageIsAdmin)}}
 			{{ctx.Locale.Tr "version"}}: