Introduce screen initialisation plugin point

At the moment when a screen is shown to a user, JIRA uses fields default values on issue creation and currently set values on all other screens (e.g. edit screens, and workflow transition screens).

Sometimes it is desirable to configure what the values that will be shown on each screen are. We can do this in several ways:

• alllow to cofnigure field defualts per screen
• introduce a new plugin type that is called before each screen initialisation
• allow to supply javascript that runs on screen load

For a sample use case see JRA-7456