[FIX] login via GitLab

[dasjoe]

• Update identityPath as API v3 was removed from Gitlab 11
• Enable mergeUsers

Closes #14307

API v3 removal: https://gitlab.com/gitlab-org/gitlab-ce/issues/36819
Without enabling mergeUsers we experience issues where users can no longer login, User with username xyz already exists

[CLAassistant]

All committers have signed the CLA.

[sampaiodiego]

thanks @dasjoe for your contribution.

we're evaluating the impact of this change, specially for those who cannot upgrade their gitlab.

[rodrigok]

@dasjoe the solution we found was:

1. Expose all string properties like identityPath as fields to be editable;
2. Change the default value of identityPath to the new API version for new installations;
3. Keep the v3 for those that are already using rocket.chat;

What do you think?

@Hudell will work on this.

[sampaiodiego]

Thanks @dasjoe .. I've merged the other PR that also updates the identityPath to v4 and added you as co-author..