Support [contribution.custom_nn] tokens in templates

Add support for processing of tokens for contribution custom fields.

The attached patch provides working beta code for this functionality. It includes changes to support processing of tokens in the form

in templates.

To Do's:
Provide tests.
Add support contribution tokens in reminder messages.
Add support for inserting tokens in templates using the drop-down.
Add support for custom field tokens for other components, eg membership, events, etc.
Limit in the UI the ability to insert custom field tokens to the components associated with that template (ie make templates aware of which components they support).

Next steps: feedback from core team on what is required for this to be included in a release. Preference: just tests, and any fixes required.