Allow homepage and logo rebranding

Fixes #2550 

Adds ability to use channel_data.json inside channel data directory to customize front page, and custom images inside an images folder.

Also adds this content for KA Lite channel.
TIL about the [bang pattern](http://git-scm.com/docs/gitignore) for `.gitignore`:
>An optional prefix "!" which negates the pattern; any matching file excluded by a previous pattern will become included again. It is not possible to re-include a file if a parent directory of that file is excluded. Git doesn’t list excluded directories for performance reasons, so any patterns on contained files have no effect, no matter where they are defined. Put a backslash ("\") in front of the first "!" for patterns that begin with a literal "!", for example, "\!important!.txt".