Enable user to not include projects when migrating groups - frontend

This is frontend issue, backend is Enable user to not include projects when migrat... (#383332 - closed)

Problem

When user migrates a group by direct transfer, on gitlab.com they cannot choose not to migrate project relations.

Proposed solution

Add a possibility for user to choose in UI when they migrate group(s) to disable migrating any projects.

User can choose for each group separately not to migrate its projects.

User can select few/all groups and choose not to migrate projects of this groups with one global toggle.

The default is migrating group(s) with projects.

Designs

For each group/row - option 3b from comment:

We should also add a note indicating that project imports is a Beta feature, where Beta links to https://docs.gitlab.com/ee/policy/alpha-beta-support.html#beta-features, something like:

Global toggle, but as above, there should be one other choice in drop down, not like in the design below where there are two option shown:

• Update documentation https://docs.gitlab.com/ee/user/group/import/, currently we have "When migrating a top-level group to GitLab.com, all its subgroups and projects are migrated too."