Fix Issue watching / unwatching on the web ui

When subscribing or unsubscribing to/from an issue on the web ui, the
request was posted to a route handled by `repo.IssueWatch`. This
function used `ctx.Req.PostForm.Get()`, erroneously.

`request.PostForm` is *only* available if `request.ParseForm()` has been
called before it. The function in question did not do that. Under some
circumstances, something, somewhere did end up calling `ParseForm()`,
but not in every scenario.

Since we do not need to check for multiple values, the easiest fix here
is to use `ctx.Req.PostFormValue`, which will call `ParseForm()` if
necessary.

Fixes go-gitea#3516.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

Author: algernon

release-notes/8.0.0/fix/3562.md

@@ -0,0 +1 @@
+Fixed a bug where subscribing to or unsubscribing from an issue in a repository with no code produced an internal server error.

routers/web/repo/issue_watch.go

@@ -46,7 +46,7 @@ func IssueWatch(ctx *context.Context) {
 		return
 	}
 
-	watch, err := strconv.ParseBool(ctx.Req.PostForm.Get("watch"))
+	watch, err := strconv.ParseBool(ctx.Req.PostFormValue("watch"))
 	if err != nil {
 		ctx.ServerError("watch is not bool", err)
 		return