refactor: migrate server to actix-http

[robjtede]

blocked on #226

actix-http is the lower level HTTP lib that powers Actix Web, it's similar in scope/goals to hyper v0.14

pros:

• –5 LoC
• we don't need to alter the routing setup
• we can use automatic HTTP/2 connections from the reverse proxy if we want to

cons:

• don't get to use the extensive middleware (first- and third-party) selection designed for Actix Web

tokei:

 Language            Files        Lines         Code     Comments       Blanks
===============================================================================
 Rust                   30         3947         3391           22          534
 |- Markdown             6           48            0           43            5
 (Total)                           3995         3391           65          539