Do some validation in search to stop weird errors

[struan]

DataError at /search/
value too long for type character varying(1000)

Mostly due to penetration attempts but still.

Relevant bit of exception is:

File "/data/vhost/[cape.mysociety.org/caps/caps/views.py](http://cape.mysociety.org/caps/caps/views.py)", line 575, in form_valid
    return self.render_to_response(context)
  File "/data/vhost/[cape.mysociety.org/caps/caps/views.py](http://cape.mysociety.org/caps/caps/views.py)", line 578, in render_to_response
    self.save_search(context)
  File "/data/vhost/[cape.mysociety.org/caps/caps/views.py](http://cape.mysociety.org/caps/caps/views.py)", line 552, in save_search
    saved_search.save()