Restful refactor of accounts/ routes (#2133)

* Add routing specs for accounts followers and following actions

* Use more restful route naming for public account follow pages

Moves two actions:
- accounts#followers to accounts/follower_accounts#index
- accounts#following to accounts/following_accounts#index

Adds routing spec to ensure prior URLs are preserved.

app/controllers/concerns/account_controller_concern.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module AccountControllerConcern
+  extend ActiveSupport::Concern
+
+  FOLLOW_PER_PAGE = 12
+
+  included do
+    layout 'public'
+    before_action :set_account
+    before_action :set_link_headers
+    before_action :check_account_suspension
+  end
+
+  private
+
+  def set_account
+    @account = Account.find_local!(params[:account_username])
+  end
+
+  def set_link_headers
+    response.headers['Link'] = LinkHeader.new(
+      [
+        webfinger_account_link,
+        atom_account_url_link,
+      ]
+    )
+  end
+
+  def webfinger_account_link
+    [
+      webfinger_account_url,
+      [%w(rel lrdd), %w(type application/xrd+xml)],
+    ]
+  end
+
+  def atom_account_url_link
+    [
+      account_url(@account, format: 'atom'),
+      [%w(rel alternate), %w(type application/atom+xml)],
+    ]
+  end
+
+  def webfinger_account_url
+    webfinger_url(resource: @account.to_webfinger_s)
+  end
+
+  def check_account_suspension
+    gone if @account.suspended?
+  end
+end