The code in `src/boot/` reads the `sidebarAppUrl` setting from
`js-hypothesis-config` JSON objects in the host page (defaulting to the
value of the build-time `__SIDEBAR_APP_URL__` template variable, if there's
no `sidebarAppUrl` setting in the host page).

`boot/` then writes `sidebarAppUrl` as the value of the `href` of a
`<link type="application/annotator+html">` element that it injects into
the host page.

Later, the code in `src/annotator/` reads this `<link>` element's `href`
in as the `config.app` setting, which it uses as the `src` of the
sidebar app's `<iframe>` that it injects into the host page.

So the variable that's named `sidebarAppUrl` / `__SIDEBAR_APP_URL__` at
boot / build time* gets renamed to `config.app` when it makes it into
the `src/annotator` code.

Fix this by renaming `config.app` in `src/annotator` to
`config.sidebarAppUrl` so that the variable is named consistently.

`*` In the build-time code there's also a `defaultSidebarAppUrl`
variable, and an `H_SERVICE_URL` environment variable, which also belong to
the same sidebarAppUrl config setting.