[feature] Add meta robots tag; allow robots to index profile card if user is Discoverable (#842)

* rework robots.txt response

* don't let robots snippet from statuses/threads

* allow robots to index if user is Discoverable

* add license text

web/template/header.tmpl

@@ -6,6 +6,7 @@
 	<meta charset="UTF-8">
 	<meta http-equiv="X-UA-Compatible" content="IE=edge">
 	<meta name="viewport" content="width=device-width, initial-scale=1.0">
+	<meta name="robots" content="{{ if .robotsMeta }}{{ .robotsMeta }}{{ else }}noindex, nofollow{{ end }}">
 	{{ if .ogMeta }}{{ if .ogMeta.Locale }}<meta name="og:locale" content="{{ .ogMeta.Locale }}">
 	{{ end }}<meta name="og:type" content="{{ .ogMeta.ResourceType }}">
 	<meta name="og:title" content="{{ .ogMeta.Title }}">

web/template/profile.tmpl

@@ -29,7 +29,7 @@
     </div>
     <h2 id="recent">Latest public toots</h2>
 	    {{ if not .statuses }}
-        <div class="nothinghere">Nothing here!</div>
+        <div data-nosnippet class="nothinghere">Nothing here!</div>
         {{ else }}
         <div class="thread">
             {{ range .statuses }}

web/template/status.tmpl

@@ -1,4 +1,4 @@
-<div class="contentgrid">
+<div data-nosnippet class="contentgrid">
 	<a href="{{.Account.URL}}" class="avatar"><img src="{{.Account.Avatar}}" alt=""></a>
 	<a href="{{.Account.URL}}" class="displayname">{{if .Account.DisplayName}}{{emojify .Account.Emojis (escape .Account.DisplayName)}}{{else}}{{.Account.Username}}{{end}}</a>
 	<a href="{{.Account.URL}}" class="username">@{{.Account.Acct}}</a>
@@ -52,4 +52,4 @@
 		<div id="favorites"><i aria-label="Favorites" class="fa fa-star"></i> {{.FavouritesCount}}</div>
 	</div>
 </div>
-<a href="{{.URL}}" class="toot-link">View toot</a>
+<a data-nosnippet href="{{.URL}}" class="toot-link">View toot</a>

web/template/thread.tmpl

@@ -1,6 +1,6 @@
 {{ template "header.tmpl" .}}
 <main>
-	<div class="thread">
+	<div data-nosnippet class="thread">
 		{{range .context.Ancestors}}
 		<div class="toot">
 			{{ template "status.tmpl" .}}