Removing hardcoded values from application

[emdupre]

In creating the developer docs (#806), I've noticed several hard-coded values in the codebase. Specifically:

• The reviewer spreadsheet ID is hard-coded in stats.rake on line 37
• The assignment of editor-in-chief is currently hard-coded in utils.rake in a weekly system (i.e. the EiC assignment cycles through several, hard-coded editors)

It would be great to extract these and add them to the config such that they can be more flexibly modified in future development !