Add template for successful submission of webmention

[philgyford]

If target and source URLs are successfully submitted via the form in WebmentionView then it returns:

return HttpResponse("Thank you, your webmention has been accepted.", status=202)

If this was replaced with a basic template then it would be possible for projects using the app to use the view, but to override both the mentions/webmention-submit-manual.html template and the new "thank you" response template, to keep appearances consistent.