GitHub importer: give feedback to the user on gists that were skipped as too big or didn't import because of other reasons

Release notes

You can now bulk-import your GitHub personal gists to GitLab personal snippets via API, however gists larger than 10 files are skipped. If any gists were skipped or did not import for any other reason, you will receive an email with the list of gists that could not be imported and reason for import failure.

Problem to solve

After GitHub Importer: API for bulk import of persona... (#371099 - closed) is completed we enable bulk-import of user personal gists via API.

In case of gists with multiple files, we:

• skip the import of anything larger than 10 files
• log an error when a gist import has to be skipped in a way we can query the frequency
• document this limit and encourage customers to manually migrate these rare gists over to projects

However, we don't give user direct feedback on gists that were skipped as too big or didn't import because of other reasons.

Proposal

After import has completed, if any gists were skipped or did not import for any other reason, send an email to the user with the list of gists that were not imported and reason for import failure.