This changeset allows ownCloud to run with pretty URLs, they will be used if mod_rewrite and mod_env are available. This means basically that the `index.php` in the URL is not shown to the user anymore.

Also the not deprecated functions to generate URLs have been modified to support this behaviour, old functions such as `filePath` will still behave as before for compatibility reasons.

Examples:
http://localhost/owncloud/index.php/s/AIDyKbxiRZWAAjP => http://localhost/owncloud/s/AIDyKbxiRZWAAjP
http://localhost/owncloud/index.php/apps/files/ => http://localhost/owncloud/apps/files/

Due to the way our CSS and JS is structured the .htaccess uses some hacks for the final result but could be worse... And I was just annoyed by all that users crying for the removal of `index.php` ;-)

For enhanced security it is recommended to configure Redis to only accept connections with a password. (http://redis.io/topics/security)

This is especially critical since Redis supports the LUA scripting language and thus a simple SSRF vulnerability (as proven in http://benmmurphy.github.io/blog/2015/06/04/redis-eval-lua-sandbox-escape/ for example) may lead to a remote code execution.

This reverts commit 7cbdd9b9.

* if you install owncloud via package it is not
  possible to skip migration tests
* this also allows to disable migration tests for
  an instance by default

* ref #16891

that I noticed while looking at some code.

* introduces config.php option 'debug' that defaults to false
* migrate DEBUG constant to config value

This will prevent detecting remote changes done in the data folder /
root storage by default. In the rare cases where the data folder is
shared with other apps/users outside ownCloud and change detection is
needed, the admin will have to set the option explicitly from now on.

Note that this doesn't affect external storages which have their own
setting in the mount options.

 * Log condition for log level increase based on conditions. Once one of these
   conditions is met, the required log level is set to debug. This allows to
   debug specific requests, users or apps

 * Supported conditions (setting `log_condition` in `config.php`):
    - `shared_secret`: if a request parameter with the name `log_secret` is set to
                this value the condition is met
    - `users`:  if the current request is done by one of the specified users,
                this condition is met
    - `apps`:   if the log message is invoked by one of the specified apps,
                this condition is met
 * fix unit test and add app log condition test

This Pull Request introduces a SabreDAV plugin that will block all older clients than 1.6.1 to connect and sync with the ownCloud instance.

This has multiple reasons:

1. Old ownCloud client versions before 1.6.0 are not properly working with sticky cookies for load balancers and thus generating sessions en masse
2. Old ownCloud client versions tend to be horrible buggy

In some cases we had in 80minutes about 10'000 sessions created by a single user. While this change set does not really "fix" the problem as 3rdparty legacy clients are affected as well, it is a good work-around and hopefully should force users to update their client