chore: enable full flake8 default rules list

[chanind]

This PR enables the full set of flake8 default rules by using extend-select and extend-ignore instead of select and ignore.

In addition, this PR makes the following notable changes:

• Simplifies the Github action to just run flake8 . so it will use the same settings as the flake8 on local
• Ignores the geom_median dir, since that's its own project, and its unclear if that should be linted by this
• ignores E402 in dashboard_runner.py and generate_dashboards.py due to those files having sys.path.append() calls above imports. It's not clear to me why those calls are there, so I just left a TODO and ignored the error
• removes # noqa statements that didn't seem necessary
• adds r to regex strings, which I think was probably the intention anyway

closes #14

[jbloomAus]

@chanind Thanks so much!