Add SEARCH_ALL_VISIBLE_TOOTS env flag

Context: https://docs.joinmastodon.org/user/network/#search

Vanilla Mastodon intentionally refuses to search outside a user's
own toots, favs, bookmarks, and mentions. This flag makes that
restricted search behavior a per-instance choice, defaulting
to the same behavior as vanilla Mastodon if the flag is absent.

.env.production.sample

@@ -276,3 +276,6 @@ MAX_POLL_OPTION_CHARS=100
 # Units are in bytes
 MAX_EMOJI_SIZE=51200
 MAX_REMOTE_EMOJI_SIZE=204800
+# Search all visible toots
+# Only relevant when elasticsearch is installed
+# SEARCH_ALL_VISIBLE_TOOTS=true

app/services/search_service.rb

@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class SearchService < BaseService
+
+  SEARCH_ALL_VISIBLE_TOOTS = ENV['SEARCH_ALL_VISIBLE_TOOTS'] == 'true'
+
   def call(query, account, limit, options = {})
     @query   = query&.strip
     @account = account
@@ -35,7 +38,9 @@ class SearchService < BaseService
   end
 
   def perform_statuses_search!
-    definition = parsed_query.apply(StatusesIndex.filter(term: { searchable_by: @account.id }))
+    statuses_index = StatusesIndex
+    statuses_index = statuses_index.filter(term: { searchable_by: @account.id }) unless SEARCH_ALL_VISIBLE_TOOTS
+    definition = parsed_query.apply(statuses_index)
 
     if @options[:account_id].present?
       definition = definition.filter(term: { account_id: @options[:account_id] })