Nudge user to provide unique name for extension on save

[twschiller]

Context

• Some users end up with identically named bricks when learning PixieBrix
• PixieBrix does not enforce name uniqueness
• The page editor generate basic names based on the current page, e.g., "My github.com button"
• User extensions (not associated with a blueprint) have a non-human-readable uuid (it's a uuid v4)

Implementation Sketch

• On Save in the Page Editor, if the this is the initial save
• Check to see if the name is already in use by a user extension
• If it is, suggest to the user to provide a different name for the extension because the name is already in use

Discuss

• We may always want to nudge the use to give a meaningful name to the extension on initial save, even if the name is not already in use

Related Issues

• For blueprints, we're solving the problem by surfacing the registry id: https://github.com/pixiebrix/pixiebrix-extension/issues/3954