Refactor repo into multiple libraries

[geoo89]

The code in parsers/common and parsers/sheets (spreadsheet -> python object) is independent and of interest independent of the rest of the repository, and this should go into its own library so that it can be used in different contexts.

The code in rapidpro/models may be of standalone interest as well (and is independent), as it provides models to create RapidPro flows from.

The flow parser from this repo in parsers/creation would then import these two as libraries.