Support manual slugs and slug verification on organization creation

[agjohnson]

The form should:

• Auto slug the name field into an editable slug field
• Throw some errors when slug doesn't happen
• For nicest UX, slug field should have live verification via and API call, but this can also be a form level error to start

Front conversations

[humitos]

More cases to consider when working on this issue:

• slugs too long
• show a warning message that even if the slug is valid may lead to other issues

(from readthedocs/readthedocs.org#10948)

[agjohnson]

Noted in 10948 that the PR build hostname is even shorter, by about 8 characters to be safe. This allows for --###### PR build postfix on the project slug.

[humitos]

Today we received this comment via support request:

When setting up the project or organization name, I accidentally put my name on it ("John Doe"), and I found it hard to change. In the end, I didn't change it because I don't know how.