Issue 309 customizable branding

This implements a build-time customization facility by which selected media and configuration elements can be substituted for the standard ones. The eligible substitutions are described by a configuration file, such that any files in a designated directory with names matching one of the entries will be copied to platform build locations, as designated in the configuration entry, at the end of the build preparations phase.

The facility, and particularly the configuration of it and its extension, are described in [the White label App Customization wiki page](https://github.com/SpiderOak/SpiderOakMobileClient/wiki/White-label-App-Customization).

It also include a collection of boring alternate icons and setting, for testing.  (I've successfully tested Android customization, but haven't been able to test the iOS build.)

Fixes #309.