Install Omeka S

Some installation requirements were not satisfied.

• Omeka requires the PHP extension mbstring, but it is not loaded.
• Omeka requires the PHP extension xml, but it is not loaded.
• An exception occurred in driver: SQLSTATE[HY000] [1045] Access denied for user 'ricdigit'@'localhost' (using password: YES)
• "/var/www/html/exhibits/files" is not a writable directory.