When session_keepalive is true (default) the heartbeat will be send as
often as the half of the session timeout value.

/* Enable maintenance mode to disable ownCloud
   If you want to prevent users to login to ownCloud before you start doing some maintenance work, 
   you need to set the value of the maintenance parameter to true. 
   Please keep in mind that users who are already logged-in are kicked out of ownCloud instantly.
*/

add options to disable the check for a working .htaccess file in data and for a working WebDAV server. This are advanced settings that are needed in special situations where our check fail and the user runs into an http timeout.

This should help make OpenSSL configuration on Windows servers easier by allowing the openssl.cnf file to be set directly in the ownCloud config, rather than in SetEnv commands that don't exist and are hard to replicate in IIS.

Remove gravatar and no-avatar functionality, prepare for default avatars even more and reword some stuff

default session_lifetime is 24hrs
recreation of session is triggered at 50% of the session life time

clarify documentation. Fixes https://github.com/owncloud/core/issues/2624 

see #2568