generate_ajax_serial() was still hard-coding a requirement for

settings.TEMPLATE_DIRS, which hasn't existed since Django 1.8.

Projects that wanted to use it needed to pull out the modern version of

this setting in order to get the function work.
Now, this function is compatible with both setups. It will

optimistically try to load from the modern TEMPLATES setting

(supporting any and all registered template engines) and falling back to

the old TEMPLATE_DIRS.
There's also a small tweak to this. If no template directories are

configured, it just sets the default serial of 0. It no longer walkes

the current directory, which is unlikely to ever be useful.

Unit tests pass.
Tested with Review Board with just a TEMPLATES setting.