Overwrite ID and updateURL for Adblock Plus for Firefox when creating a non-release build

[tlucas]

Background

In #6371 we want to self-host our extension for firefox again. For that to work, we need to provide a different extension ID and a different update-URL in the manifest.

What to change

Change mainfest.json.tmpl in buildtools to generate the following entries in the extensions manifest ONLY when no release-build is created, only when building for Firefox ( build.py build -t gecko ):

{
   "applications": {                                                             
     "gecko": {                                                                  
       "id": "devbuild@adblockplus.org",
       ...
     }                                                                           
   },
   ...
}

Note that "​devbuild@adblockplus.org" is an arbitrary id, chosen by us.

[abpbot]

A commit referencing this issue has landed:
​Issue 6383 - introduce switchable app_id