Refactor exports controller (#1567)

* Add basic coverage for settings/exports controller

* Remove unused @account variable from settings/exports controller

* Add coverage for download export actions

* Remove deprecated `render :text` in favor of `send_data` for csv downloads

* Add model to handle exports

* Use Export class in settings/exports controller

* Simplify settings/exports controller methods

* Move settings/export to more restful routes

app/models/export.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+require 'csv'
+
+class Export
+  attr_reader :accounts
+
+  def initialize(accounts)
+    @accounts = accounts
+  end
+
+  def to_csv
+    CSV.generate do |csv|
+      accounts.each do |account|
+        csv << [(account.local? ? account.local_username_and_domain : account.acct)]
+      end
+    end
+  end
+end