Evaluate new upstream Copier/platform-cli features

[doshitan]

When initially implementing the Copier version of this template, the functionality wasn't there to loop over a part of the template and re-render files with different parameters. We worked around this with custom support in the platform-cli by basically treating template-infra as two separate templates, a "base" that gets installed once and an "app" that gets re-installed for every application (the looping part).

That's potentially changed with a new (currently unreleased) feature in Copier: copier-org/copier@557c0d6

Related CLI ticket: navapbc/platform-cli#15

We should evaluate the new functionality more and determine which path to take:

1. Continue the "base" and "app" distinctions, perhaps further clarifying with Move template content into sub-directories #818
2. Use the new dynamic file structure support to treat template-infra as a single template with an array of applications
   1. We need to ensure we are able to specify different parameters for each application (like the port, etc), that we can prompt for the app parameters, and that the app parameters get saved sanely in the answers file.
   2. Build migration support from current "base" and "app" parts to re-unify into single answers file (assuming that's how the feature works upstream)