Automation for BeeWare Pull Requests board

[glasnt]

In the theme of the BeeWare Planning project board (for issues), build and automate the BeeWare Pull requests for pull requests.

The purpose of this is to help improve the triager happy path per beeware/beeware#618 to have a readily assessible list of work to be reviewed.

[glasnt]

Limitation: boards only support one main project (without payment). Work around: automation.

Steps required:

• Copying the new-issue template from non-toga boards (e.g. https://github.com/beeware/beeware.github.io/blob/main/.github/workflows/new-issue.yml) to a new-pr.yml workflow
• Creating checks that mark (for example) PRs that pass all CI checks as "Ready" (or similar)
• Adding a project tab view to show all PRs that are "Ready"

[glasnt]

There are a number of notes in #399 about the limitations of that implementation that should be referenced later.

[glasnt]

An alternative implementation is a poll-based method like https://github.com/poseidon/wait-for-status-checks

Example implementation: https://github.com/c-ares/c-ares/blob/main/.github/workflows/all-checks.yml

Having an additional step to add our label could work here. It also allows us to have explicit excludes (known flaky tests).

[freakboy3742]

See review comment on beeware/toga-chart#344 for discussion about potential issues with the poll-based method.