Extend issue template yaml engine (#29274)

Add new option:

`visible`: witch can hide a specific field of the form or the created
content afterwards

It is a string array witch can contain `form` and `content`. If only
`form` is present, it wont show up in the created issue afterwards and
the other way around. By default it sets both except for markdown

As they are optional and github don't have any similar thing, it is non
breaking and also do not conflict with it.

With this you can:
- define "post issue creation" elements like a TODO list to track an
issue state
- make sure to have a checkbox that reminds the user to check for a
thing but dont have it in the created issue afterwards
- define markdown for the created issue (was the downside of using yaml
instead of md in the past)
 - ...

## Demo

```yaml
name: New Contribution
description: External Contributor creating a pull

body:
- type: checkboxes
  id: extern-todo
  visible: [form]
  attributes:
    label: Contribution Guidelines
    options:
      - label: I checked there exist no similar feature to be extended
        required: true
      - label: I did read the CONTRIBUTION.MD
        required: true
- type: checkboxes
  id: intern-todo
  visible: [content]
  attributes:
    label: Maintainer Check-List
    options:
      - label: Does this pull follow the KISS principe
      - label: Checked if internal bord was notifyed
# ....
```
[Demo
Video](https://cloud.obermui.de/s/tm34fSAbJp9qw9z/download/vid-20240220-152751.mkv)

---
*Sponsored by Kithara Software GmbH*

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>
(cherry picked from commit 77e29e0c39392f142627303bd798fb55258072b2)

modules/issue/template/unmarshal.go

@@ -128,9 +128,18 @@ func unmarshal(filename string, content []byte) (*api.IssueTemplate, error) {
 			}
 		}
 		for i, v := range it.Fields {
+			// set default id value
 			if v.ID == "" {
 				v.ID = strconv.Itoa(i)
 			}
+			// set default visibility
+			if v.Visible == nil {
+				v.Visible = []api.IssueFormFieldVisible{api.IssueFormFieldVisibleForm}
+				// markdown is not submitted by default
+				if v.Type != api.IssueFormFieldTypeMarkdown {
+					v.Visible = append(v.Visible, api.IssueFormFieldVisibleContent)
+				}
+			}
 		}
 	}