Warn the user if the issue numbering between source and repository won't match

[IQAndreas]

There is no way to specify an issue ID when creating a new issue, so unless precautions are taken, new issues will have a different id than the ones they were created in the image of (which means that relative links, example: #3, won't work in the target repo).

I want to notify the user if:

• Only some issues are imported, meaning there will be "gaps" causing issue ids to be off by that amount
• The target repository already has issues in it

I also want to detect if the id of the new issue's id will exactly match up even if imported (say, if new issues are added to the source repository, and those new issues are added after the original "import").