[apache] Add `Open Web App Manifest` configs

An Open Web App manifest file (having the extension `.webapp`):

* must be served with a `Content-Type` header of
  `application/x-web-app-manifest+json` (this is currently not enforced
  by Firefox but is enforced by the Firefox Marketplace while, Firefox
  OS only checks this if the origin of the page where the user triggers
  the install is different from the origin of the app itself)

* must be UTF-8 encoded in order to be submitted to the Firefox
  Marketplace

* should not be cached (the expires headers for the manifest are usually
  ignored by Firefox but, just to be sure):

  "Webapps.jsm <http://mxr.mozilla.org/mozilla-central/source/dom/apps/src/Webapps.jsm>
  sets INHIBIT_CACHING <http://mxr.mozilla.org/mozilla-central/source/netwerk/base/public/nsIRequest.idl#141>
  when it downloads the manifest, which prevents Firefox from caching it
  so, in theory the Firefox app runtime won't take expires headers into
  consideration. However, INHIBIT_CACHING doesn't prevent the runtime
  from using an entry in the cache if it exists so, if you load the
  manifest yourself, f.e. to see what it looks like, while developing
  your app, Firefox might cache it, and Webapps.jsm might then get the
  cached version.
  ...
  Webapps.jsm is the implementation of the DOMApplicationRegistry that
  is responsible for managing app records, including their manifests"

                            ~ thanks to @mykmelez for the explanation ~

* is a JSON file so, it can be compressed (the manifest validator
  <https://marketplace.firefox.com/developers/validator>, doesn't seem
  to mind)

* doesn't need any of the extra headers (e.g.:`Content-Security-Policy`,
  `X-UA-Compatible`, etc.)

Reference: https://developer.mozilla.org/en-US/docs/Apps/Manifest