Refactor i18n to locale (#20153)

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

templates/base/footer_content.tmpl

@@ -1,9 +1,9 @@
 <footer>
 	<div class="ui container">
 		<div class="ui left">
-			{{.i18n.Tr "powered_by" "Gitea"}}
+			{{.locale.Tr "powered_by" "Gitea"}}
 			{{if (or .ShowFooterVersion .PageIsAdmin)}}
-				{{.i18n.Tr "version"}}:
+				{{.locale.Tr "version"}}:
 				{{if .IsAdmin}}
 					<a href="{{AppSubUrl}}/admin/config">{{AppVer}}</a>
 				{{else}}
@@ -11,8 +11,8 @@
 				{{end}}
 			{{end}}
 			{{if and .TemplateLoadTimes ShowFooterTemplateLoadTime}}
-				{{.i18n.Tr "page"}}: <strong>{{LoadTimes .PageStartTime}}</strong>
-				{{.i18n.Tr "template"}}
+				{{.locale.Tr "page"}}: <strong>{{LoadTimes .PageStartTime}}</strong>
+				{{.locale.Tr "template"}}
 				{{if .TemplateName}} {{.TemplateName}}{{end}}: <strong>{{call .TemplateLoadTimes}}</strong>
 			{{end}}
 		</div>
@@ -22,16 +22,16 @@
 			{{end}}
 			<div class="ui language bottom floating slide up dropdown link item">
 				{{svg "octicon-globe"}}
-				<div class="text">{{.i18n.LangName}}</div>
+				<div class="text">{{.locale.LangName}}</div>
 				<div class="menu language-menu">
 					{{range .AllLangs}}
-						<a lang="{{.Lang}}" data-url="{{AppSubUrl}}/?lang={{.Lang}}" class="item {{if eq $.i18n.Lang .Lang}}active selected{{end}}">{{.Name}}</a>
+						<a lang="{{.Lang}}" data-url="{{AppSubUrl}}/?lang={{.Lang}}" class="item {{if eq $.locale.Lang .Lang}}active selected{{end}}">{{.Name}}</a>
 					{{end}}
 				</div>
 			</div>
-			<a href="{{AssetUrlPrefix}}/js/licenses.txt">{{.i18n.Tr "licenses"}}</a>
+			<a href="{{AssetUrlPrefix}}/js/licenses.txt">{{.locale.Tr "licenses"}}</a>
 			{{if .EnableSwagger}}<a href="{{AppSubUrl}}/api/swagger">API</a>{{end}}
-			<a target="_blank" rel="noopener noreferrer" href="https://gitea.io">{{.i18n.Tr "website"}}</a>
+			<a target="_blank" rel="noopener noreferrer" href="https://gitea.io">{{.locale.Tr "website"}}</a>
 			{{template "custom/extra_links_footer" .}}
 		</div>
 	</div>