Pass file paths to render task instead of file content.

# Purpose
Previously, most add-ons passed file contents to the Celery render task
as a string, which has the potential to flood the message broker with
data. This patch saves files to a temporary directory on disk and passes
the file paths to Celery instead. Temp files are deleted after rendering
is complete.

# Changes
* Render task accepts temp file path rather than content
* Render task deletes temp file after rendering

# Side effects
* Must add `MFR_TEMP_PATH` to settings
* Must ensure permissions on temp path

Thanks @icereval for writing this with me.