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

Cargo.toml

@@ -24,7 +24,6 @@ rocket = "0.4.2"
 rocket_contrib = { version = "0.4.2", features = ["json"] }
 rocket_i18n = { git = "https://github.com/Plume-org/rocket_i18n", rev = "e922afa7c366038b3433278c03b1456b346074f2" }
 rpassword = "4.0"
-runtime-fmt = "0.4.0"
 scheduled-thread-pool = "0.2.2"
 serde = "1.0"
 serde_json = "1.0"