Drop the 'slides' processing scripts.

Processing new recordings in the 'slides' format is no longer supported
in this release of BigBlueButton.

The playback files are retained, so that existing slides recordings
from previous BigBlueButton versions can be played.
Fix file permissions

A bunch of these files were incorrectly marked as executable.
Rename playback.html to index.html

In the new recording layout, it's now the index file for the directory
containing the standalone recording.
Adapt playback js for running in recording directory
Adapt writing.js for running in recording directory