Refactor render system (#32492)

There were too many patches to the Render system, it's really difficult
to make further improvements.

This PR clears the legacy problems and fix TODOs.

1. Rename `RenderContext.Type` to `RenderContext.MarkupType` to clarify
its usage.
2. Use `ContentMode` to replace `meta["mode"]` and `IsWiki`, to clarify
the rendering behaviors.
3. Use "wiki" mode instead of "mode=gfm + wiki=true"
4. Merge `renderByType` and `renderByFile`
5. Add more comments

----

The problem of "mode=document": in many cases it is not set, so many
non-comment places use comment's hard line break incorrectly

modules/templates/util_render.go

@@ -94,8 +94,9 @@ func (ut *RenderUtils) RenderCommitBody(msg string, metas map[string]string) tem
 	}
 
 	renderedMessage, err := markup.RenderCommitMessage(&markup.RenderContext{
-		Ctx:   ut.ctx,
-		Metas: metas,
+		Ctx:         ut.ctx,
+		Metas:       metas,
+		ContentMode: markup.RenderContentAsComment,
 	}, template.HTMLEscapeString(msgLine))
 	if err != nil {
 		log.Error("RenderCommitMessage: %v", err)
@@ -116,8 +117,9 @@ func renderCodeBlock(htmlEscapedTextToRender template.HTML) template.HTML {
 // RenderIssueTitle renders issue/pull title with defined post processors
 func (ut *RenderUtils) RenderIssueTitle(text string, metas map[string]string) template.HTML {
 	renderedText, err := markup.RenderIssueTitle(&markup.RenderContext{
-		Ctx:   ut.ctx,
-		Metas: metas,
+		Ctx:         ut.ctx,
+		ContentMode: markup.RenderContentAsTitle,
+		Metas:       metas,
 	}, template.HTMLEscapeString(text))
 	if err != nil {
 		log.Error("RenderIssueTitle: %v", err)