Identify maximum usable URL length for Zoopla/Rightmove, build feature to error before loading

[beveradb]

To avoid Zoopla/Rightmove etc. getting errors from users' IP addresses and blocking them (?), and in general just to provide a better user experience, we should detect what the most complex encoded polyline we can send to them in the URL is, and only ever open the new tab if the URL we're about to load will not error.

e.g. write quick script which tests this using curl to see how long the param has to be before we get a 5XX response instead of a 200?