Add Issues to Projects

### Summary

I added an /issues endpoint to close #85. The /projects endpoint has been updated to return the project's issues, closing #86. Detailed additions below:

#### app.py
- Added Issue Model
- Added issues relationship to Project
- Updated Project.asdict to return issues
- Added /issues and /issues/<int:id> routes to query and return issues

#### run_update.py
- Fixed bug where script would fail if organization's github_url was invalid (**fails silently and continues for now**)
- Add get_issues function to get all github issues from projects about to be added (**verifying keep=True**)
- Added save_issue_info function that takes a list of dicts with issue information and saves them to the DB.
- Modified main to call get_issues after it finishes adding each organization

#### run_update_test.py
- Modified tests to consider the new http requests to github so they wouldn't fail anymore

#### index.html
- Added /issues information to index page.