Since FIXTURE_DIR_NAME is the name of the folder that contains the view
and tests modules of the webhook and another folder called "fixtures" that
store the fixtures, it is more appropriate to call it WEBHOOK_DIR_NAME,
especially when we want to refer to the view module using this variable.
Note that the documentation cannot fully use our macros, because
Uptime Robot requires an & of the end of the URL, because of how it
passes its payload.
Fixes#13854. Fixes#13939.