Deprecate lang-specific flag, default to iso column on input

[kassyray]

Our current pipeline runs with a language specific flag, specifying an input file that contains the clients requiring notices in the language specified.

However, sometimes we receive data all in one file, and are required to map schools to language with a separate file. Then, we can run the pipeline as usual.

This feature request involves deprecating the language specific flag, defaulting to an iso column in the client data that specifies the language that a given client requires. The pipeline functions such that the preprocessed json includes a client-specific language code in the client details. The CLI flag patches this for all clients, but with an input file, we could have this done.

This approach may also touch on the following issues: #110 and #68