ScriptRunner event handlers loading slowly

Event handlers load slowly when you have a mixture of admin and per repository items setup.

This is because for each item we get a specific list of events depending on which of the two it is.

This means calling it for an admin item then calling it for a per repository item will evict the admin item events from the cache and the list of events will be recalculated.

We should split the events that are to be cached into per repo and admin level ones and cache these, so the list of events will only ever be calculated on the first call.