Adds feature requested in Mantis 0013180: Url template custom field

By choosing custom field type 'Template', you can define a template value with a placeholder ({0}) in the 'Default value' field. Upon entering a value for this custom field, it will be inserted in the template's placeholder. Links will be interpreted.

Screenshots:
Field definition: http://grab.by/bPp0
Edit field: http://grab.by/bPp2
Display: http://grab.by/bPp6
History: http://grab.by/bPp8