Add support for remote interact (#519)

Add support for remote interaction ([this thing](https://eldritch.cafe/users/Barmaid/remote_follow) in mastodon)

- [x] create the endpoint dispatching remote interactions to local pages
- [x] add this endpoint to web-finger
- [x] propose remote interaction when following and not connected
- [x] propose remote interaction when liking/sharing and not connected
- [x] propose remote interaction when commenting and not connected
- [x] fetch posts/comments we don't know but remote interaction was requested for ?

templates/users/header.rs.html

@@ -32,7 +32,7 @@
             <a class="u-url" href="@user.ap_url"></a>
         }
 
-        @if ctx.2.clone().map(|u| u.id != user.id).unwrap_or(false) {
+        @if ctx.2.clone().map(|u| u.id != user.id).unwrap_or(true) {
             <form class="inline" method="post" action="@uri!(user::follow: name = &user.fqn)">
             @if follows {
                 <input type="submit" value="@i18n!(ctx.1, "Unsubscribe")">