Add project "description" field to project_config.json files.
Do fabrication of config.json

and remove the default one from the repository.

Lots of sundry tweaks along the way.
Fabricate project manifests incorporating brand specifics.

- Create new 'custom/brand_elements.json' to provide the structure
  transformation data.

  It's analogous to 'custom/elements.json', in that defines the process
  rather than the data being processed. It should changes only rarely,
  eg to specify transformations for new platform manifests.

- Move the project config and manifests to ".template"-suffixed versions,
  and .gitignore the fabricated, non-suffixed versions.

  In order to make transformation of the SpiderOak-Info.plist file easy
  using node elementtree, add a distinguishing attribute to the target
  string tag.

  So:
      <string>com.spideroak.android</string>
  is now:
      <string this="CFBundleIdentifier">com.spideroak.android</string>
  I don't expect that this would disrupt the build, and haven't noticed
  any problems, but I'm not sure. If necessary, it would not be hard to
  add something to omit the attribute in the produced version.