Refactor usage of view models

[thekaveman]

As shown in #1436, adding or moving around copy driven by translations requires changes in multiple places:

• models, to reference the PO entries
• PO files, to hold the translations
• view view models, to get the data into the view

We need to look into how to simplify this, ideally cutting out extraneous view models that already have a corresponding model, and sending that directly to the template instead.

Plan

After some different iterations, this is the approach we landed on:

• Move all translated strings out of models, and as close to the templates (HTML) as possible
• Use fields in the models to reference template variants when needed (e.g. for TransitAgency and EligibilityVerifier)
• Simplify the views as much as possible: remove usage of viewmodels, create basic dictionary context that has exactly the fields needed for that view/page

Acceptance Criteria

The following viewmodels and helpers are removed:

• viewmodels.Button Refactor: buttons #1562
  • viewmodels.Button.agency_contact_links Refactor: agency links #1521
  • viewmodels.Button.home Refactor: home button #1522
  • viewmodels.Button.link Refactor: agency links #1521
  • viewmodels.Button.login Refactor: Eligibility start page #1498
  • viewmodels.Button.logout Refactor: sign out button #1534
  • viewmodels.Button.primary Refactor: buttons #1562
  • viewmodels.Button.outline_primary Refactor: buttons #1562
  • viewmodels.Button.previous_page Refactor: buttons #1562
• viewmodels.Icon Refactor: remove icon/media item view models #1542
• viewmodels.MediaItem Refactor: remove icon/media item view models #1542
• viewmodels.Modal Refactor: remove Modal viewmodel #1504
  • viewmodels.AgencySelector Refactor: remove Modal viewmodel #1504
• viewmodels.Page Refactor: remove Page viewmodel #1563
• viewmodels.ErrorPage Refactor: error pages #1536
  • viewmodels.ErrorPage.user_error Refactor: error pages #1536
  • viewmodels.ErrorPage.server_error Refactor: error pages #1536
  • viewmodels.ErrorPage.not_found Refactor: error pages #1536
• viewmodels.PaymentProcessor Refactor: Enrollment index #1502
• viewmodels.TransitAgency Refactor: remove Transit Agency viewmodel #1541

[thekaveman]

We could move most of what view models are doing to using Django template tags. This seems like a good approach because:

• Removes need for view model classes, can pass needed data to each tag or get it from context
• Moves us closer to the concept of "components", will simplify and clarify templates and help reuse
• Likely simplifies view functions as well
• Tags should be straightforwardly testable

Django makes two types available OOTB:

• simple tags return a string to be wrapped by other HTML in a template
• inclusion tags use their own named template file, like includes do in the HTML

Django also has a way to write more advanced custom tags, probably more than we need for now.

[machikoyasuda]

Putting these notes here:

There are 3 places in the new Veteran flow app that requires logic in the views with knowledge of EligibilityVerifier-specific information:

• Eligibility Start (show 3 bullets vs. 4 bullets in the first identity MediaItem)
• Sign Out link (show Sign Out link or not -- could have some sort of EligibilityVerifier.supports_signout boolean on the model) Refactor sign-out link logic for AuthProvider's that support it #1456
• Use Button.login() (which sets the button's CSS ID to login and specifies a hard-coded fallback text of Login.gov). The Veteran flow would use a regular button without any special CSS ID or fallback text.

[thekaveman]

We have a pretty solid plan:

• Move all translated strings out of models, and as close to the templates (HTML) as possible
• Use fields in the models to reference template variants when needed (e.g. for TransitAgency and EligibilityVerifier)
• Simplify the views as much as possible: remove usage of viewmodels, create basic dictionary context that has exactly the fields needed for that view/page

And some different pieces of this implementation:

• Refactor: verifier radio template #1489
• Refactor: Eligibility start page #1498
• Refactor: Enrollment index #1502
• Refactor: remove Modal viewmodel #1504
• Refactor: Eligibility index #1517

[thekaveman]

Blocked: #1563 (comment)