Persist gists import failures

What does this MR do and why?

If a gist fails to be imported, we will save the failure in the database to be able to present any failed imports to the user in the future.

This implements Track gists import failure.

Screenshots or screen recordings

How to set up and validate locally

1. Use GitLab API to try to import one or more GitHub gists with more than 10 files.
2. In rails console make sure that related failures have been saved to the database:

ImportFailure.where(user: user) # 'user' - GitLab user who initiated the gists import

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.