Add sharing endpoints H

[eyurtsev]

The goal is to produce a shareable URL that will allow anyone with the access to the URL to use the extractor.

Backend:

1. When an extractor is shared, its content is copied to a new table and it becomes immutable. This content includes associated examples. It's assigned a new uuid, and it can be fetched from GET /s/{uuid} endpoint.

2. When an extractor is shared it's assigned as shared UUID. The extractor is accessible via the shared UUID, GET endpoints do not return internal UUIDs. Content includes examples.

UI:

Main UX question decide what the sidebar does when one navigates to the shared extractor

[eyurtsev]

merged