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 ?

static/css/_forms.scss

@@ -132,4 +132,29 @@ form.new-post {
 
 .button + .button {
   margin-left: 1em;
+}
+
+.split {
+  display: flex;
+  justify-content: space-between;
+
+  & > * {
+    flex-grow: 1;
+    max-width: 40%;
+  }
+}
+
+header.center {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  background: transparent;
+  opacity: 1;
+  font-weight: normal;
+  text-align: left;
+
+  > * {
+    margin-left: 0;
+    margin-right: 0;
+  }
 }