Editor preview support for external renderers (#23333)

Remove `[repository.editor] PREVIEWABLE_FILE_MODES` setting that seemed
like it was intended to support this but did not work. Instead, whenever
viewing a file shows a preview, also have a Preview tab in the file
editor.

Add new `/markup` web and API endpoints with `comment`, `gfm`,
`markdown` and new `file` mode that uses a file path to determine the
renderer.

Remove `/markdown` web endpoint but keep the API for backwards and
GitHub compatibility.

## ⚠️ BREAKING ⚠️

The `[repository.editor] PREVIEWABLE_FILE_MODES` setting was removed.
This setting served no practical purpose and was not working correctly.
Instead a preview tab is always shown in the file editor when supported.

---------

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/swagger/v1_json.tmpl

@@ -951,6 +951,38 @@
         }
       }
     },
+    "/markup": {
+      "post": {
+        "consumes": [
+          "application/json"
+        ],
+        "produces": [
+          "text/html"
+        ],
+        "tags": [
+          "miscellaneous"
+        ],
+        "summary": "Render a markup document as HTML",
+        "operationId": "renderMarkup",
+        "parameters": [
+          {
+            "name": "body",
+            "in": "body",
+            "schema": {
+              "$ref": "#/definitions/MarkupOption"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/responses/MarkupRender"
+          },
+          "422": {
+            "$ref": "#/responses/validationError"
+          }
+        }
+      }
+    },
     "/nodeinfo": {
       "get": {
         "produces": [
@@ -17991,7 +18023,7 @@
           "type": "string"
         },
         "Mode": {
-          "description": "Mode to render\n\nin: body",
+          "description": "Mode to render (comment, gfm, markdown)\n\nin: body",
           "type": "string"
         },
         "Text": {
@@ -18005,6 +18037,33 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "MarkupOption": {
+      "description": "MarkupOption markup options",
+      "type": "object",
+      "properties": {
+        "Context": {
+          "description": "Context to render\n\nin: body",
+          "type": "string"
+        },
+        "FilePath": {
+          "description": "File path for detecting extension in file mode\n\nin: body",
+          "type": "string"
+        },
+        "Mode": {
+          "description": "Mode to render (comment, gfm, markdown, file)\n\nin: body",
+          "type": "string"
+        },
+        "Text": {
+          "description": "Text markup to render\n\nin: body",
+          "type": "string"
+        },
+        "Wiki": {
+          "description": "Is it a wiki page ?\n\nin: body",
+          "type": "boolean"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "MergePullRequestOption": {
       "description": "MergePullRequestForm form for merging Pull Request",
       "type": "object",
@@ -20835,6 +20894,12 @@
         "type": "string"
       }
     },
+    "MarkupRender": {
+      "description": "MarkupRender is a rendered markup document",
+      "schema": {
+        "type": "string"
+      }
+    },
     "Milestone": {
       "description": "Milestone",
       "schema": {