Allow anyone to resolve an issue

[jacekv]

Currently an issue works like the following:
Random user opens issue -> Owner of repository works on problem, submits solution and resolves issue. Opener can close the issue

The same process does not work if the owner of a repo opens an issue. A random user is not able to resolve the issue, only the owner.

This should be changed, so that a random user is able to resolve an issue if the owner opened the issue.

This requires that a user is able to fork the repository, resolve the problem, push, and create a PR.
Once the issue is resolved, the changes are merged into the owners repository.

[jacekv]

Blocked by #25 and #26