Add support to bbs2gh migrate-repo to only generate the archive on …

…the BBS server (#723)

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Closes #729 

This adds support for calling `bbs2gh migrate-repo` with a combination
of args that will cause it to tell BBS to generate the archive and do
nothing else.

This can be useful in a few scenarios, but for right now the goal is to
support a customer that uses different proxy servers depending on if
they are talking to an internal server (i.e. BBS) or an external server
(i.e. Azure/GitHub). With this change you will be able to call
`migrate-repo` once to trigger the BBS archive generation, then switch
up your proxy settings and call `migrate-repo` a 2nd time to tell it to
upload to azure and call github.com.

- [x] Did you write/update appropriate tests
- [x] Release notes updated (if appropriate)
- [x] Appropriate logging output
- [x] Issue linked
- [x] Docs updated (or issue created)

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Co-authored-by: Arin Ghazarian <aringhazarian@github.com>