[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.

plugins/ClientSideShorten/locale/ClientSideShorten.pot

@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PACKAGE VERSION\n"
 "Report-Msgid-Bugs-To: \n"
-"POT-Creation-Date: 2019-06-08 18:20+0100\n"
+"POT-Creation-Date: 2019-08-14 14:44+0100\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -23,7 +23,7 @@ msgid "\"text\" argument must be specified."
 msgstr ""
 
 #. TRANS: Plugin description.
-#: ClientSideShortenPlugin.php:66
+#: ClientSideShortenPlugin.php:67
 msgid ""
 "ClientSideShorten causes the web interface's notice form to automatically "
 "shorten URLs as they entered, and before the notice is submitted."