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 #3516.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
