Support Issue forms and PR forms (#20987)

* feat: extend issue template for yaml

* feat: support yaml template

* feat: render form to markdown

* feat: support yaml template for pr

* chore: rename to Fields

* feat: template unmarshal

* feat: split template

* feat: render to markdown

* feat: use full name as template file name

* chore: remove useless file

* feat: use dropdown of fomantic ui

* feat: update input style

* docs: more comments

* fix: render text without render

* chore: fix lint error

* fix: support use description as about in markdown

* fix: add field class in form

* chore: generate swagger

* feat: validate template

* feat: support is_nummber and regex

* test: fix broken unit tests

* fix: ignore empty body of md template

* fix: make multiple easymde editors work in one page

* feat: better UI

* fix: js error in pr form

* chore: generate swagger

* feat: support regex validation

* chore: generate swagger

* fix: refresh each markdown editor

* chore: give up required validation

* fix: correct issue template candidates

* fix: correct checkboxes style

* chore: ignore .hugo_build.lock in docs

* docs: separate out a new doc for merge templates

* docs: introduce syntax of yaml template

* feat: show a alert for invalid templates

* test: add case for a valid template

* fix: correct attributes of required checkbox

* fix: add class not-under-easymde for dropzone

* fix: use more back-quotes

* chore: remove translation in zh-CN

* fix EasyMDE statusbar margin

* fix: remove repeated blocks

* fix: reuse regex for quotes

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_json.tmpl

@@ -16584,6 +16584,35 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "IssueFormField": {
+      "description": "IssueFormField represents a form field",
+      "type": "object",
+      "properties": {
+        "attributes": {
+          "type": "object",
+          "additionalProperties": {},
+          "x-go-name": "Attributes"
+        },
+        "id": {
+          "type": "string",
+          "x-go-name": "ID"
+        },
+        "type": {
+          "$ref": "#/definitions/IssueFormFieldType"
+        },
+        "validations": {
+          "type": "object",
+          "additionalProperties": {},
+          "x-go-name": "Validations"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
+    "IssueFormFieldType": {
+      "type": "string",
+      "title": "IssueFormFieldType defines issue form field type, can be \"markdown\", \"textarea\", \"input\", \"dropdown\" or \"checkboxes\"",
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "IssueLabelsOption": {
       "description": "IssueLabelsOption a collection of labels",
       "type": "object",
@@ -16608,6 +16637,13 @@
           "type": "string",
           "x-go-name": "About"
         },
+        "body": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/IssueFormField"
+          },
+          "x-go-name": "Fields"
+        },
         "content": {
           "type": "string",
           "x-go-name": "Content"