New source form errors are not cleared after input values have been fixed

[alex-ketch]

If an input field contains an error, the form is not able to be re-submitted even after the issue has been addressed.
The only way to fix it is to reload the page.

To reproduce:

1. Navigate to a Project on the Hub
2. Add a new GitHub source
3. Enter stenci.la as the Url
4. Click Create Source button
5. See Not a valid GitHub URL. error
6. Fix the Url value by entering https://github.com/stencila/hub/blob/master/README.md
7. Click Create Source button
8. 🐛 Nothing happens, error is still shown
9. Reload page, and enter correct Url from step 6
10. Source is successfully created

As part of this issue, we should verify that this bug is not present on other forms.