Add support for placeholders with information from parent issue

[Stefan Glase]

As a user I want to use the parent key and parent summary in my subtasks.

So we need to add support for placeholders in summary and text fields (custom fields / description). Possible values are:

• @{key} for the key of the parent issue
• @{summary} for the summary of the parent issue

Considerations

How do we deal with the typical @inherit possibility? Will users understand that I can either inherit the whole part or use @{summary} to construct a new value using the parent value as a part of it?

Examples

- documentation for @{summary}
- add @{key} to release notes

Tasks

• implement keyword replacement for summary / description / custom field text and textarea
• add documentation
• test behavior with maximum field length

[Stefan Glase]

We will first implement #82 and see whether this already solves all requirements.