e2e: Proposal Edit mock structure.

[victorgcramos]

Part of #2642.

Proposal Edit tests are still using the online api approach. We need to migrate it to the new mock structure introduced by (#2624).

We currently don't need new test suites, the main focus here is to first migrate and then code more tests using this new structure.

Tips and Tricks

• Only mock requests related to Proposal Edit page
• Run only edit test file: yarn cy:run -- --spec "cypress/e2e/proposal/edit.js"
• Setup the API environment for all possible routes reached by the tests suite using a beforeEach statement
• Check on politeiawww's logs to check if all requests were mocked properly
• Use proposal/list.js tests as reference

[vibros68]

I am on it