Add feature to view admin_audit log files

[Abijeet]

Add a LogFileProvider class that identifies the available log files It checks if the admin_audit app is enabled, and fetches it's logfile path.

Additionally, add ability for user to select the file they want to view. The "Download logs" button now downloads the currently selected log file.

Tested by enabling and disabling the admin_audit app

TODO: Update the commands to also use the chosen logfile

Fixes: nextcloud#318 Signed-off-by: Abijeet [email protected]