Standardize staggered view json input.

[RichardeJiang]

[RichardeJiang]

As a first step, moved all template image metadata into the json, and now inside the StaggeredGridView, dynamically load info from json instead of hard-code it into the dart file.

[RichardeJiang]

Note that we may not want to close the ticket until the actual images to be used are loaded into the project, and all use cases have been finalized.