remove dependency on runtime-fmt (#773)

Per the issue, "runtime-fmt uses perma-unstable rust APIs and is
therefore susceptible to breakage".

This replaces the two calls to rt_format! with .replace() and drops the
dependency.

Fixes #769

src/main.rs

@@ -6,8 +6,6 @@ extern crate gettext_macros;
 #[macro_use]
 extern crate rocket;
 #[macro_use]
-extern crate runtime_fmt;
-#[macro_use]
 extern crate serde_json;
 #[macro_use]
 extern crate validator_derive;

src/routes/posts.rs

@@ -643,7 +643,7 @@ pub fn remote_interact_post(
         .and_then(|blog| Post::find_by_slug(&rockets.conn, &slug, blog.id))?;
     if let Some(uri) = User::fetch_remote_interact_uri(&remote.remote)
         .ok()
-        .and_then(|uri| rt_format!(uri, uri = target.ap_url).ok())
+        .map(|uri| uri.replace("{uri}", &target.ap_url))
     {
         Ok(Redirect::to(uri).into())
     } else {

src/routes/user.rs

@@ -201,15 +201,14 @@ pub fn follow_not_connected(
         if let Some(uri) = User::fetch_remote_interact_uri(&remote_form)
             .ok()
             .and_then(|uri| {
-                rt_format!(
-                    uri,
-                    uri = format!(
+                Some(uri.replace(
+                    "{uri}",
+                    &format!(
                         "{}@{}",
                         target.fqn,
                         target.get_instance(&rockets.conn).ok()?.public_domain
-                    )
-                )
-                .ok()
+                    ),
+                ))
             })
         {
             Ok(Redirect::to(uri).into())