Choose namespace and name on Github imports

Lets the user choose a namespace and name on Github imports

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14971

There are four different states depending on the user privileges. If the user is able to select a namespace it should be prompt for it as the following screenshot:

If the user has no groups but is able to create a group, a similar drop down will appear which will include the user namespace plus the repo group (selecting the latter, will create a new namespace if it doesn't exist yet).

Similarly, if the user has groups and is also able to create groups, the same drop down will appear but will also include the repo group (if it does not exist already).

The third option will be when the user has no groups and has no privileges to create a group. In this case there's no option and the default namespace is shown:

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