chore(web): disable browserstack on non-web-specific builds

Because BrowserStack tests are unreliable, I am disabling them for any
builds that are not specifically targeting web. Two ways that the tests
can be enabled:

1. Include '(web)' in the PR title
2. Include the label test-browserstack

This only impacts builds in the CI test environment, not anything else.

See build-utils-ci.inc.sh for helper functions for accessing PR
metadata, and build-utils-ci.test.sh for unit tests for this
functionality.