docs(CHANGELOG): fix 0.12.0 commits and issues links

Hello,

I just had a problem with the dropdown changes in 0.12.0.
I was trying to find out what the problem was. So I looked into this CHANGELOG.md and couldn't see the related commits and issues due to broken links (Page not found).

Hope this isn't a bad fix.

```
angular-ui/bootstrap/commit/{SHA} → https://github.com/angular-ui-bootstrap/commit/{SHA}
angular-ui/bootstrap/issues/{SHA} → https://github.com/angular-ui-bootstrap/issue/{issue-id}
```