Introduce mechanism to periodically verify unresolved emails

CollectOSS maintains an unresolved emails table for skipping contributor/commit emails that could not be resolved to actual github users.

https://github.com/augurlabs/augur/pull/3711 introduced a PR to automatically clear these emails, however that is not the right approach.

id rather do something like:

- add a column for number of attempts made to resolve that contributor, and only ignore the contributor if its greater than some threshold
- introduce a task to explicitly go through and re-attempt to resolve unresolved contributors (since github downtime is a known way that this list can pile up)

This issue is intended to document the need to have *some* mechanism for this list to be pruned-down by the system, because as-is, anything that ends up in the list (such as maybe by a github API fault) will be there until it is manually cleared.

Start by locating the unresolved emails table and the contributor email-resolution entry point, then review how GitHub API failures are recorded. Done means the system retries unresolved contributors through an explicit mechanism, applies an attempt threshold, and prunes entries without requiring manual clearing.

Written by the indexing model from the issue text.