Yunus/issue improvements

- Subscribed_emails list is implemented for issues.
- Email commit author as well

@RISCfuture @RickCSong 
When an issue is updated, the page is partially refreshed. The refreshed section is the issue-wrapper (show-wrapper.html.slim) which contains the _show partial and _edit partial. If updating the issue fails (for ex, invalid parameters), I don't want the _show partial to reflect the changes that couldn't be persisted.

I can probably explain better in person if this is not clear.