Add feature to view admin_audit log files

[ghost]

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/disabling admin_audit app, toggling "Log file"

[joshtrichards]

I haven't had an opportunity to look closely at the code yet, but I like the idea.

Some of this is based on #476, correct?

This would certainly further address #318 / #129 w/o forcing the admin to use consolidated logging (see nextcloud/documentation#10604).

P.S. Looks like one of your commits isn't signed so the tests can't run. A rebase to master probably a.good idea while you're at it. :-)