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)

Cargo.toml

@@ -29,6 +29,7 @@ serde = "1.0"
 serde_json = "1.0"
 serde_qs = "0.5"
 shrinkwraprs = "0.2.1"
+syntect = "3.3"
 validator = "0.8"
 validator_derive = "0.8"
 webfinger = "0.4.1"