Add multiple line-items/vendors

[rememberlenny]

Proposals require a way to depict having multiple line-items and multiple vendors.

Reference doc: https://docs.google.com/document/d/1_gjq_3m16Ug7_JcfsV28Lt4dCQTDnEhvWVFofbNaw4g/edit

Reference issue: 18F/C2: Issue #1151

[pkarman]

There are multiple ways to implement a one-to-many model relationship like this.

1. The least invasive approach would be to follow the example of related steps, and create a proposals.parent_id column so that proposals can be grouped together into a set. The assumption there is that there is enough difference amongst the client_data attribute values on each proposal that we aren't simply duplicating N values on each proposal in the set. If that assumption were false, that would be a disadvantage of this approach. The advantages include: minimal db change, no need to affect existing data, maximum flexibility for different client_data values on each line, ability to approve/disapprove steps on lines independently (line-item veto), and no need to duplicate aggregate values (like total amount) since they can be calculated dynamically.
2. Create a new client_data_items table with a client_data_id column. These rows would each represent a single line item. Advantages include: "true-er" representation of business case. Disadvantages: potentially duplicated data in client_data and client_data_items, need to keep them in sync.
3. Create a JSON column on client_data called items. This gives us the most flexibility in terms of the data model, since the internal structure of that column would be managed entirely in code. Advantages: near-zero data duplication. Disadvantages: managing data in code means we cannot rely on db constraints.