Add hints for special Wiki pages (#17283)

* Add UI hints to create a Wiki sidebar and footer

options/locale/locale_en-US.ini

 wiki.reserved_page = The wiki page name '%s' is reserved.
 wiki.pages = Pages
 wiki.last_updated = Last updated %s
+wiki.page_name_desc = Enter a name for this Wiki page. Some special names are: 'Home', '_Sidebar' and '_Footer'.
 
 activity = Activity
 activity.period.filter_label = Period:

routers/web/repo/wiki.go

 	if !ctx.Repo.Repository.HasWiki() {
 		ctx.Data["title"] = "Home"
 	}
+	if ctx.FormString("title") != "" {
+		ctx.Data["title"] = ctx.FormString("title")
+	}
 
 	ctx.HTML(http.StatusOK, tplWikiNew)
 }

templates/repo/wiki/new.tmpl

 			<div class="field {{if .Err_Title}}error{{end}}">
 				<input name="title" value="{{.title}}" autofocus required>
 			</div>
+			<div class="help">
+				{{.i18n.Tr "repo.wiki.page_name_desc"}}
+			</div>
 			<div class="ui top attached tabular menu previewtabs" data-write="write" data-preview="preview">
 				<a class="active item" data-tab="write">{{.i18n.Tr "write"}}</a>
 				<a class="item" data-tab="preview" data-url="{{$.Repository.APIURL}}/markdown" data-context="{{$.RepoLink}}">{{$.i18n.Tr "preview"}}</a>