Use BitBucket UUID instead of username

[stephen-smith]

https://github.com/davido/gerrit-oauth-provider/blob/90b66bbf374fe4575b829eb41d940556060fda31/src/main/java/com/googlesource/gerrit/plugins/oauth/BitbucketOAuthService.java#L98

Grabs the "username" and "display_name" and might fix up the old numeric ids.

But, BitBucket allows users to change these fields, so UUIDs need to be used in prevent spoofing: https://confluence.atlassian.com/bitbucket/rest-apis-222724129.html#RESTAPIs-uuid-mainUniversallyUniqueIdentifier(UUID)

[davido]

If we will do that change, we would need some kind of data migration.

[stephen-smith]

Because we did not do this change, #120 and #127 are currently preventing some users from logging in.

[davido]

So yes, now the problem is even more serious, because we have to change first to API v2 to start getting the data again. We have this PR, that I closed without merging, because the canonical repository is googlesource.com.