Add support for docsearch appId for custom search deployments.

[abernix]

From my verbose commit message:

e4ed72b (Jesse Rosenberger, 34 minutes ago)

   This only seems to be important to me for the moment because of my need to
   test search improvements.  Typically, Algolia docsearch runs their own free
   deployment for open-source projects.  This is done through a public
   configuration repository available at
   https://github.com/algolia/docsearch-configs.

   However, in order to test these configurations, it's necessary to run your
   own deployment through their paid service (or a free trial!).  Their public
   service defaults to their own app, but when using the paid service, it's
   necessary to specify the `appId`, which this new code allows.

And a related enhancement:

58fac8d (Jesse Rosenberger, 35 minutes ago)
   Only inject docsearch JavaScript when docsearch is enabled.

   While this won't be necessary for much longer since we'll enable it
   everywhere (hopefully), there's no sense in loading extra unused JavaScript
   on repositories that may choose to not have search, or just don't have it
   configured yet.