new Settings for migration when assignee users don't exist and S3 isn't used for attachment storage

[bensons]

Created settings for clearing issue assignees, bypassing S3 bucket upload, saving issue
attachments locally, and rewriting attachment URLs in issue content. Fixed problem downloading
attachments with international characters by encoding attachment download URIs.

[ujos]

Very cool commit. I have a few notes from my side if I could:

1. Would be great if you could split clearIssueAssignment, attachment URI encoding and S3 stuff so they can be applied separately.
2. useS3 flag conflicts with overrideURL flag. I would use overrideURL only if useS3 is FALSE

[ujos]

I believe would be better to have two commits:

1. One commit for keepLocal+overrideURL+useSe
2. Another commit for clearIssueAssignment

[ujos]

The space character removed between ) {... For cleaner commit would be better to keep original formatting

[spruce]

@bensons Any comments regarding the review?

[phil-blain]

I've used the code in this PR to transfer the images to a hidden ref in the repo and use a relative link to have them show up in the issues, i.e. something like ../blob/<hash>/path/to/image.png?raw=true (cf. github/docs#17479).

Since the PR does not merge cleanly anymore, here is an up-to-date version if it helps anyone (I don't have time right now to clean it up and submit a proper PR): master...phil-blain:fixes. The only thing I did not reapply is the clearIssueAssignment setting.

I've also removed the use of the githubRepoId in the relative path as this solves a chicken-and-egg problem where if you don't know the repo ID before running the conversion, you can't use it in the overrideURL setting...