Require conversation resolution before merging

Problem to solve

Protected branches in GitLab are implemented differently than in GitHub. For example, in GitHub there are some protected rules that you can apply on one or a few branches that match an expression, while those rules exist in GitLab as a general project settings for all branches. In this issue we make the mapping, so if a protected rule is defined in GH, and similar rule exists in GitLab general settings, we will configure the migrated GitLab project with this rule as a general settings, and this rule will be applied to all branches.

Purposed solution

 Scenario: "Require conversation resolution before merging"
    Given "Require conversation resolution" before merging is defined as a branch protected rule in GitHub
    When a user imports this repository to GitLab
    Then the settings All threads must be resolved in the GitLab project settings will be enabled 

Add documentation of this mapping.

Technical details

Get default branch protection rule from Github API. Look at required_conversation_resolution value and put it into projects only_allow_merge_if_all_discussions_are_resolved attr.