Release based on Gerrit plugin API 2.13.6

New features:

• @nayobix added Facebook provider
• @TuvaevAndrey added GitLab provider

Note:

Prefix user id with oauth provider name issue was fixed.
To migrate the legacy user id, fix-legacy-user-id property must be set to
true for all providers.

OpenID 2.0 identifier mapping is discontinued issue was fixed.
As pointed out in the Google OpenID Migration spec:

  Support for the OpenID 2.0 identifier mapping described above will remain in effect until January 1, 2017.

Linking of legacy OpenID identifier is ignored now. Warning is issued if you still try to activate the mapping.

It worth noting that the build toolchain was migrated to Bazel.