Make plots.py significantly (4x) faster

[andylolz]

plots.py grinds to a near halt when it gets to this bit:

-> publisher_types
-> activities_per_publisher_type

This PR massively speeds up those two steps, by avoiding loading and parsing the same file(s) hundreds of times.

[andylolz]

Without this fix, python plots.py takes 1:22:25 (user + sys) to complete on my machine.
With this fix, python plots.py takes 0:21:55 (user + sys) to complete on my machine.

[andylolz]

Worth noting that this bug was introduced in 24a381e. That change looks innocuous if you assume that data.ckan_publishers is a normal dict. If that were the case, then sure, there’s a lookup inside a loop that could be outside the loop, but lookups are fast so who cares.

But data.ckan_publishers isn’t a normal dict – data.ckan_publishers[publisher] loads a JSON file from disk, and (if the publisher has changed registry ID) potentially loads a bunch of other stuff too.

So this PR just reverts that bit of 24a381e.

[Ocre42]

Thanks for the input

[andylolz]

Cool! Thanks for merging, @Ocre42!