Installer: configurable videos folder

install.php now takes the videos path (form field / --videos= CLI flag),
persists it into config.php, and creates the directory if missing.
config.example.php default changed from a dev path to /srv/courses.

config.example.php

@@ -5,7 +5,7 @@
 //   /srv/media/courses/
 //     Course 1/Intro/Video1.mp4
 //     Course 2/Module 3/Advanced/lesson.mp4
-define('VIDEOS_ROOT', '/home/harwinder/Development/coursehub/videos');
+define('VIDEOS_ROOT', '/srv/courses');
 
 define('DB_HOST', '127.0.0.1');
 define('DB_PORT', 3306);