Add support for omniauth-jwt provider

To configure a custom OmniAuth strategy we need to add the gem to the Gemfile and install it. This is currently not possible if GitLab EE is installed using the Omnibus packages. JWT is a common and simple authentication method that would allow many external systems be used for authentication.

Proposal

• Add the omniauth-jwt provider omniauth-jwt
• Add configuration examples to gitlab.yml and gitlab.rb

Links / references

https://docs.gitlab.com/ce/integration/omniauth.html#using-custom-omniauth-providers