HTTP Security plug: rewrite &csp_string/0

- Directives are now separated with ";" instead of " ;",
according to https://www.w3.org/TR/CSP2/#policy-parsing
the space is optional
- Use an IO list, which at the end gets converted to a binary as
opposed to ++ing a bunch of arrays with binaries together and joining
them to a string. I doubt it gives any significant real world advantage,
but the code is cleaner and now I can sleep at night.
- The static part of csp is pre-joined to a single binary at compile time.
Same reasoning as the last point.

test/plugs/http_security_plug_test.exs

@@ -67,7 +67,7 @@ defmodule Pleroma.Web.Plugs.HTTPSecurityPlugTest do
 
       [csp] = Conn.get_resp_header(conn, "content-security-policy")
 
-      assert csp =~ ~r|report-uri https://endpoint.com; report-to csp-endpoint;|
+      assert csp =~ ~r|report-uri https://endpoint.com;report-to csp-endpoint;|
 
       [reply_to] = Conn.get_resp_header(conn, "reply-to")