Automate updates for list of searx instances (#3)

* Create nightly update workflow for instances

A nightly GitHub Actions CI workflow has been added to fetch new
instances of supported services within Farside.

Currently only Searx is supported, but obviously others could be added
if there are similarly easy ways to fetch and filter instances
programmatically.

services.json has also been updated with the initial results of the
workflow script.

* Set headers for every HTTPoison request

This serves as a workaround for bot blocking via filtron.

* Expand filtering of searx instances

New filter enforces:
- No Cloudflare
- Good TLS config
- Good HTTP header config
- Vanilla instances or forks
- Instances with 100% search success

lib/farside/instances.ex

@@ -3,6 +3,7 @@ defmodule Farside.Instances do
   @update_file Application.fetch_env!(:farside, :update_file)
   @services_json Application.fetch_env!(:farside, :services_json)
   @service_prefix Application.fetch_env!(:farside, :service_prefix)
+  @headers Application.fetch_env!(:farside, :headers)
 
   def sync() do
     File.rename(@update_file, "#{@update_file}-prev")
@@ -21,7 +22,7 @@ defmodule Farside.Instances do
       System.get_env("FARSIDE_TEST") ->
         :good
       true ->
-        case HTTPoison.get(url) do
+        case HTTPoison.get(url, @headers) do
           {:ok, %HTTPoison.Response{status_code: 200}} ->
             # TODO: Add validation of results, not just status code
             :good