Optimize timetable rendering

[adamcik]

Profiling of the code has shown that most time is spent within [source:trunk/plan/templates/schedule_table.html]. The current version of this code has three nested for loops within the template code, unfortunately for-loops within django-templates are rather slow due the amount of overhead introduced by django's template features.

Currently the performance is not really an issue, so this task is by no means a priority. However if one where to attempt to solve this issue here are a few possible solutions:

• Create a template tag that uses a faster templating system internally, eg. Jinja (alternatively convert all templates to Jinja)
• Only construct the cell using template code, use pure-python for the loops in the table taking special care to escape everything to avoid XSS. This should probably be implemented with a custom templatetag.

[adamcik]

Timing experiments seem to indicate that the slowness is primarily in the django template system. I'm looking into switching to jinja which claims to be faster.