transactions sms endpoint

[lizzyaustad]

This should function similarly to the MEMBERS keyword texts. If a co-op leader sends a text with keyword 'TRANSACTIONS', send back a list of all transactions with state=='initiated' that belong to users in the leader's co-op.

You may want to also add a co_op_id field onto the transactions table to make the second part of this easier. If so, make sure to also add a transactions_users table, update the sample_db inputs, and update the add_transaction method to include the member's co_op_id.