Drop database views (alt. implementation)

[jdavcs]

Alternative implementation for #15876

This is a straightforward refactoring of HistoryDatasetCollectionJobStateSummary's implementation with minimal change:

• The view is replaced with a subquery (ref: example of such mapping in SA's docs)
• The structure of the original view's query is left unchanged, but the query itself is implemented in SQLAlchemy's SQL Expression Language.
• The model.views directory is dropped.

NOTE: The part of the query that enumerates the job states (aggregate sum columns) is now generated dynamically based on members of model.Job.states. This was done as a safeguard against missing a state (assertion added) or listing a non-existent state (e.g. the original view's query had a bug: the state deleted_new has been replaced with deleting, which may have resulted in incorrect counts.)

(Rationale for dropping views: see #15876)

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[jdavcs]

API test failures are relevant.

[jdavcs]

Closing in favor of #15876