Allow to inherit values from other fields

[codescape]

As a user I want to be able to inherit values from the parent issue to other fields of the newly created subtasks. For example a story with the custom user field Developer should be able to be inherited to assign the subtasks.

Possible solution would be something like this:

- new subtask using the value of the story's custom field Developer
  assignee: @inherit(customfield(Developer))
- new subtask using the assignee of the story for a custom field Developer
  customfield(Developer): @inherit(assignee)

This is a customer feature request:

• https://codescape.atlassian.net/browse/CASUP-101
• https://codescape.atlassian.net/browse/CASUP-128

Tasks

• decide about syntax
• implement
• update documentation for syntax

[codescape]

Current existing customer requests are limited to pulling specific field values into the summary of the newly created subtasks. This would make implementing this feature easier if we limit it to summary and description first.

- Designer @inherit(customfield(Designer)) needs to create a mockup with priority @inherit(priority)