Create an integration test with kinto

[dstaley]

We should create an integration test that confirms the master branch of kinto can build the admin with the PR. It should test that the admin builds, and that it loads correctly in a browser.

[leplatrem]

We have something similar in kinto-dist: https://github.com/mozilla-services/kinto-dist/blob/9f8094f84b6241621712a18ec201465cf9dead21/tests/test_integration.py#L23-L28
but in the particular case of Kinto/kinto#2646 it didn't detect the error after submitting the login form..

[alexcottner]

We're kinda doing this now, or at least I think we've got the spirit of it.

We have the browser test that will run against the latest kinto-admin release for each PR. (recent example)

For ensuring the build process works, we have CI checks on each PR (example) and a single-server admin build is automatically attached to each release. See the current release and the action log as examples.