Edition form performance improvements part 1

First part of delivering [this story](https://www.agileplannerapp.com/boards/173808/cards/5697).

This speeds up the admin edition forms by caching the data used to tag editions. In this pull request, the following taggable elements have been optimised: Organisations; Topics; and Topical events.

There are two parts to the optimisation:

1. Caching a data structure for the label/ID pairs
2. Caching the generated select/option form elements where possible.

Even with just those three associations optimised, benchmarks are showing anything from 10% to 50% improvement in page generation time, mainly down to the faster generation of the organisation select fields. World location news articles are the only exception, showing marginal improvement because they cannot be associated with "organisations", only "worldwide organisations".