validate project name as the user types

In our recent user study, the project creation failure due to upper case letter (#1058) happened again.

In addition to fixing this issue, I'd like to suggest adding a UI that prevents this from happening, **before** hitting 'Finish' and seeing the 'Cannot Save Setting' warning message.

One option would be showing an error message as the project name gets typed in, similar to password strength verification. Another option would be applying a helpful constraint, meaning that the text field only accepts lowercase letter and underscore.

Cc: @InMatrix @galeyang

Start at the project-creation UI and the project-name text field, then trace the Finish action and the existing “Cannot Save Setting” warning. Confirm which validation behavior is preferred, implement feedback before Finish, and verify that invalid uppercase names are prevented or clearly rejected while valid names still create projects.

Written by the indexing model from the issue text.