Update `logAdminViewer/deleteFiles` to use a POST w/JSON body

[amcclain]

This endpoint takes filenames to be deleted as a param via params.list('filenames')

If deleting a large number of log files, the generated URL query string overflows and you get an error about instanceId not found (which I believe is only b/c that param gets clipped).

We should issue this command - and any similar - as a POST w/JSON body. Will require Hoist React update to use XH.postJson() and then here in the controller read via parseRequestJSON()

[amcclain]

While in the client side code, let's also add proper masking while the delete is running.....