Refactor `BasePage` methods to consolidate `SavedRun` and `PublishedRun` retrieval logic into `get_sr_pr` + caching

[devxpy]

Q/A checklist

• [ ] If you add new dependencies, did you update the lock file?

  poetry lock --no-update

• [ ] Run tests

  ulimit -n unlimited && ./scripts/run-tests.sh

• [ ] Do a self code review of the changes - Read the diff at least twice.
• [ ] Carefully think about the stuff that might break because of this change - this sounds obvious but it's easy to forget to do "Go to references" on each function you're changing and see if it's used in a way you didn't expect.
• [ ] The relevant pages still run when you press submit
• [ ] The API for those pages still work (API tab)
• [ ] The public API interface doesn't change if you didn't want it to (check API tab > docs page)
• [ ] Do your UI changes (if applicable) look acceptable on mobile?
• [ ] Ensure you have not regressed the import time unless you have a good reason to do so. You can visualize this using tuna:

  python3 -X importtime -c 'import server' 2> out.log && tuna out.log

  To measure import time for a specific library:

  
  $ time python -c 'import pandas'

---

Executed in 1.15 secs fish external usr time 2.22 secs 86.00 micros 2.22 secs sys time 0.72 secs 613.00 micros 0.72 secs

To reduce import times, import libraries that take a long time inside the functions that use them instead of at the top of the file:
```python
def my_function():
    import pandas as pd
    ...

Legal Boilerplate

Look, I get it. The entity doing business as “Gooey.AI” and/or “Dara.network” was incorporated in the State of Delaware in 2020 as Dara Network Inc. and is gonna need some rights from me in order to utilize my contributions in this PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Dara Network Inc can use, modify, copy, and redistribute my contributions, under its choice of terms.

[nikochiko]

imo we should also remove is_update_mode. having a flag is much more confusing than just having separate methods separate methods or an if branch at the very end.

we can do something like

def _save_or_update_and_redirect(self, title: str, notes: str, published_run: PublishedRun | None):
  if published_run and self.can_user_edit_published_run(published_run):
    version = published_run.add_version(...)
  else:
    pr = self.create_published_run(...)

  raise gui.RedirectException(pr.get_app_url())

and in the publish modal,

if pressed_save:
  self._save_or_update_and_redirect(title=title, notes=notes)

[nikochiko]

or this can be something like - if published_run was passed then add a new version and redirect. or create a new PR instead