Support to migration "everything" from Github Enterprise to Gitlab EE

Description

EE prospect or customer who is now using Github Enterprise would like to have an utility to migrate everything from Github EE to Gitlab EE including projects/repos, groups, issues, PRs, Wikis, users with one click.

At this moment, if the GL admin tries to import from GHE by OAuth with the GHE admin account, s/he might be able to see all projects. But the namespaces may need to be checked or changed manually which might be tedious.

Users need to be migrated as well as the membership of groups.

Proposal

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Links / references

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Documentation blurb

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/cc: @brodock @dbalexandre @JobV