Add alt text to logo (go-gitea#19892)

The recommended way is to use the name of the organisation followed
by "logo". however, since this is my first contribution, I am not
entirely sure, whether this is the best approach here.

The organisation is different from the organisation you can create as
part of the application. Instead, it is more related to the site
hosting the instance. Plus, I don't know how to best handle it when
the logo image is swapped out. Therefore, I use plain "Logo" and hope
that the person visiting the site has enough context.

Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

options/locale/locale_en-US.ini

@@ -2,6 +2,7 @@ home = Home
 dashboard = Dashboard
 explore = Explore
 help = Help
+logo = Logo
 sign_in = Sign In
 sign_in_with = Sign In With
 sign_out = Sign Out

templates/base/head_navbar.tmpl

@@ -1,7 +1,7 @@
 <div class="ui container" id="navbar">
 	<div class="item brand" style="justify-content: space-between;">
 		<a href="{{AppSubUrl}}/" data-content="{{if .IsSigned}}{{.i18n.Tr "dashboard"}}{{else}}{{.i18n.Tr "home"}}{{end}}">
-			<img class="ui mini image" width="30" height="30" src="{{AssetUrlPrefix}}/img/logo.svg">
+			<img class="ui mini image" width="30" height="30" src="{{AssetUrlPrefix}}/img/logo.svg" alt="{{.i18n.Tr "logo"}}">
 		</a>
 		<div class="ui basic icon button mobile-only" id="navbar-expand-toggle">
 			<i class="sidebar icon"></i>