Change how custom fields define and load javascript

Currently, all custom fields are told to ensure their javascript is loaded in the atl.general context. The disadvantages of this approach are:

• Custom fields will only ever work in pages in the atl.general context – in other words, not admin and other areas.
  • For this reason, the quick-create dialog, for instance, cannot work in any section outside of atl.general (see JRA-36519).
• The code to run custom fields will always be loaded, even when it doesn't need to (as in, the fields it should affect aren't used or present on the page)
• The dump-it-into-a-context approach makes the implementation of plugins like the issue collector difficult – it has to load all JS assets for the atl.general context, rather than just the assets required to make the issue creation form work.

We should build a better way to package up and deliver the assets required to make a custom field work.