Add authorization support for Gitlab projects (#630)

* Add support for gitlab projets

* Add group membership in state

* Use prefixed allowed groups everywhere

* Fix: remove unused function

* Fix: rename func that add data to session

* Simplify projects and groups session funcs

* Add project access level for gitlab projects

* Fix: default access level

* Add per project access level

* Add user email when missing access level

* Fix: harmonize errors

* Update docs and flags description for gitlab project

* Add test with both projects and groups

* Fix: log error message

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Fix: make doc a markdown link

* Add notes about read_api scope for projects

* Fix: Verifier override in Gitlab Provider

This commit fixes a bug caused by an override of the Verifier value from *ProviderData inside GitlabProvider struct

* Fix: ensure data in session before using it

* Update providers/gitlab.go

Co-authored-by: Nick Meves <nick.meves@greenhouse.io>

* Rename gitlab project initializer

* Improve return value readbility

* Use splitN

* Handle space delimiters in set project scope

* Reword comment for AddProjects

* Fix: typo

* Rework error handling in addProjectsToSession

* Reduce branching complexity in addProjectsToSession

* Fix: line returns

* Better comment for addProjectsToSession

* Fix: enrich session comment

* Fix: email domains is handled before provider mechanism

* Add archived project unit test

* Fix: emails handling in gitlab provider

Co-authored-by: Wilfried OLLIVIER <wollivier@bearstech.com>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>
Co-authored-by: Nick Meves <nick.meves@greenhouse.io>