Support for issue & patch templates

[erak]

Objectives

Support structured issue / patch creation that is in line with project-specific requirements. Template files contain valid markdown that is then pasted into the description of an issue or a patch. The template for a patch could look like this:

<!--## Prerequisites

- First, many thanks for taking part in the community. We really appreciate that.
- We realize there is a lot of information requested here. We ask only that you do your best to provide as much information as possible so we can better help you.
...

*Delete the above section and the instructions in the sections below before submitting*
-->

## Describe your changes

## Issue ticket number and link

## Checklist before requesting a review
- [ ] I have performed a self-review of my code
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics?
- [ ] Will this be part of a product update? If yes, please write one phrase about this update.

Specification

• rad issue: read description template from .radicle/ISSUE_TEMPLATE
• rad patch: read description template from .radicle/PATCH_TEMPLATE

Note: It would need some more thinking about how to deal with multiple issue types, such as bugs, feature requests etc. Github stores them in separate files and the template is picked based on the reporters selection via the UI.