Standardize variable naming conventions for all the ways we refer to invest models

[davemfish]

This a task for feature/plugins

Throughout invest & workbench code, we need to refer to model names in various ways:

1. the module name (i.e. 'natcap.invest.carbon')
2. an identifier ('carbon')
3. an even shorter alias for the identifier, for some models ('cbc')
4. the official title ('Carbon Storage and Sequestration')

If we always use the same variable names when storing these values, it's easier to reason about the code.

@emlys is doing a bunch of this standardization already on the natcap.invest side in #1532 , which will merge into feature/plugins. The variable names used are:

1. pyname
2. model_id
3. model_alias
4. model_name

The Workbench (starting at payloads in ui_server.py, actually) should adopt these same conventions.