ci: run github actions on pull requests

[alxndrsn]

Closes #1308

There is some subtlety here:

• to allow personal development branches, exceptions can be included. As @ktuite names branches consistently, I've included that exception here. For other contributors, they could pick a naming scheme and add to the config
• i don't think soak tests need to run on every PR, so they're restricted to master, and also can be triggered manually from https://github.com/getodk/central-backend/actions

What has been done to verify that this works as intended?

It's not always 100% possible to test github actions config without merging it.

Why is this the best possible solution? Were any other approaches considered?

See #1308 for alternative suggestions(s).

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No.

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

No.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced

[lognaturel]

Won't this end up with duplicate runs for the branches that are marked as exceptions? Why not one of the alternatives in the issue?