Merge branch 'hotfix/3.1.2'
Handle all possible errors when uploading files (see #5934)
Update the `Database_Result` object when updating a model (see #6070)
Fix the version 3.1 changelog
Drop the database query cache (see #6070)
Return an empty model collection instead of `null` (see #6147)
Restore the `User::getData()` method (see #6146)
Revert "Return an empty model collection instead of `null` (see #6147)"

This reverts commit 4cb82f0e7d02ec268021200f88f970ffc8b07d5f.
Throw an execption in `Model::__callStatic()` if the method is unknown (see #6160)
Fix endless recursion in `loadDataContainer()` (see #6145)
Fix two issues in the `Database_Result` class
Add a PDO driver
Remove the error suppressors from the MySQLi classes
Remove the MySQL driver and update the install tool
Restore the MySQL driver for reasons of backwards compatibility
Restore persistent connection support
Merge branch 'feature/database-improvements' into develop
Do not exit after a template has been output (see #5570)
Apply the width and height to the audio player as well (see #6114)
Replace insert tags in the details view of the listing module (see #6120)