Update Bitbucket PR author login

[atefehmohseni]

Currently Bitbucket PR author login field uses user's nickname which is not unique. I suggest to use accountID or UUID instead as explained here:
https://developer.atlassian.com/cloud/bitbucket/bitbucket-api-changes-gdpr/

[bradrydzewski]

We have an ID field that is meant to hold the unique identifier. Let's use this instead for the author account id.

	User struct {
		ID      string
		Login   string
		Name    string
		Email   string
		Avatar  string
		Created time.Time
		Updated time.Time
	}

		Author: scm.User{
+			ID:     from.Author.AccountID,
			Login:  from.Author.Nickname,
			Name:   from.Author.DisplayName,
			Avatar: from.Author.Links.Avatar.Href,
		},

EDIT as an aside, the Login is currently being set to the Nickname. The Login field should only be set to the username, and if no username is available, it should be empty. We should probably revert this because it will result in a bug for anyone that uses it.

		Author: scm.User{
-			Login:  from.Author.Nickname,
			Name:   from.Author.DisplayName,
			Avatar: from.Author.Links.Avatar.Href,
		},

[atefehmohseni]

Done.