Add optional external rate limit file.

[dracos]

For https://github.com/mysociety/sysadmin/issues/1916

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 75.23%. Comparing base (43abf6f) to head (ddaa49e).

Files	Patch %	Lines
mapit/ratelimitcache.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
- Coverage   75.27%   75.23%   -0.04%     
==========================================
  Files          47       47              
  Lines        2811     2815       +4     
==========================================
+ Hits         2116     2118       +2     
- Misses        695      697       +2     

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[neprune]

looks good to me

[dracos]

Actually doing same thing in the mapit.mysociety.org repo instead.