Syntax highlighting (#691)

* Syntax highlighting mostly... Exists.

* Add dependency to dockerfile

* Handle non-existent languages better

* Make the default a bit nicer

* Improve highlighting. Clean up function

* Add dark theme, add the comment scope to the allowed classes

* update build env

* Address review comments

* Use find_syntax_by_token which produces the desired behavior

* Change flat_map into flatten
(commit cargo.lock)

plume-common/src/lib.rs

@@ -20,6 +20,7 @@ extern crate shrinkwraprs;
 extern crate serde_derive;
 #[macro_use]
 extern crate serde_json;
+extern crate syntect;
 extern crate tokio;
 
 pub mod activity_pub;