[CORE] Move core plugins to a new modules directory

For reference (raised by rozzin in IRC):

* http://foldoc.org/module
* http://foldoc.org/library
* http://foldoc.org/plugin

As noted by XRevan86, modules are not necessarily non-essential.
As we will keep the modules directory in GS root [therefore, near to
plugins/], it is evidenced the difference between both.

This is a simple yet fundamental structural change. It doesn't change
functionality but makes clearer the way we understand GNU social's
internals.

modules/AuthCrypt/locale/cs/LC_MESSAGES/AuthCrypt.po

@@ -0,0 +1,23 @@
+# Translation file for GNU social - the free software social networking platform
+# Copyright (C) 2015 - 2019 Free Software Foundation, Inc http://www.fsf.org
+# This file is under https://www.gnu.org/licenses/agpl v3 or later
+# 
+# Translators:
+msgid ""
+msgstr ""
+"Project-Id-Version: GNU social\n"
+"Report-Msgid-Bugs-To: \n"
+"POT-Creation-Date: 2015-02-02 17:47+0100\n"
+"PO-Revision-Date: 2015-02-05 17:44+0000\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: Czech (http://www.transifex.com/gnu-social/gnu-social/language/cs/)\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Language: cs\n"
+"Plural-Forms: nplurals=3; plural=(n==1) ? 0 : (n>=2 && n<=4) ? 1 : 2;\n"
+
+#. TRANS: Plugin description.
+#: AuthCryptPlugin.php:157
+msgid "Authentication and password hashing with crypt()"
+msgstr ""