Move database access code out of galaxy.util

[jdavcs]

This removes database access code from the galaxy.util package. Package tests currently pass because such code is invoked indirectly, through trans.sa_session with trans passed as an argument. However, refactoring towards SA 2.0 (replacing the legacy query object with select) exposes this and tests fail (we have to from sqlalchemy import select, and sqlalchemy is not available to the galaxy.util package; discovered in #16434).

There were 2 such instances in the util package:

1. The get_user_by_username function is moved as a standalone function into managers.users. (this is currently failing 2 tests -- I'm investigating)

2. The dbkeys module is moved into the managers package.

   The data access code in dbkeys can be easily extracted into 2 functions which can be moved to managers.hdas. However, the calling code from dbkeys would still have to be moved out of the util package, otherwise it would have to import from managers. Is there a better way to rearrange this?

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]

Package test failure unrelated. Ready for review.