fix(bitbucketcloud)!: Use AccountID as username instead of Nickname

[maxbrunet]

I want to be able to identify the user running the workflow and do checks like #308 (comment), and nickname is not appropriate:

Note that while we are introducing a nickname field, it will not be effective for identifying user accounts, because nicknames are not guaranteed to be globally unique. If you do store user data, we strongly encourage you to use Atlassian account IDs as the identifier for your data.

https://developer.atlassian.com/cloud/bitbucket/bitbucket-api-changes-gdpr/

I also clean up username from the test data, since it has been removed on 29 April 2019.

I suppose PullRequest.Author is only informative, so it should be fine to keep Nickname there.

For reference, the change from Username to Nickname was made in #502.

[maxbrunet]

I do not know all the places where User.Username is used, but policies.owners.users could be broken for some Bitbucket Cloud users for example.

Only the user's username, UUID, and accound ID can be used to identify a user via the Bitbucket Cloud REST API:

• For users who created their account before the deprecation, their nickname was initialized to the value of their username, so in this case it should work for them.
• For users who created their account recently, their nickname is initialized to the value of their display name, so it should not work for them.

And a user can change their nickname independently from their username.

[maxbrunet]

I changed my mind about PullRequest.Author, I think it makes sense to identify the user uniquely there as well, it is exposed as the PULL_AUTHOR environment variable. For a future usage example as in an RBAC mechanism defining whose PR can be auto-planned.