WebIDE: Migrate Create New File/Folder modal to use GlModal

The New File/Folder modal currently uses the old Deprecated modal. Migrate it to use the new GlModal.

Reason for change: GlModal adds modals as a child to body, whereas DeprecatedModal adds modal as a child to the container itself. When a modal is added in the container, it inherits the styles of its container, which causes a lot of problems with the dark theme.