Do not overwrite user-defined contract name on upload of metadata

[cmichi]

This is something that users were confused by at the workshops last week:

• Go to https://contracts-ui.substrate.io/instantiate
• Type a "Contract Name" (e.g. "Player")
• Upload a bundle
• The "Contract Name" is replaced by whatever name the bundle contains

I suggest that if a user already entered a "Contract Name", the uploaded bundle should not override it. Just retain the already entered contract name in the input field.