Examples with hidden lines should be editable

# Problem

For this issue, there are two types of code blocks:

- Read-only ones, defined via ` ```rust `
- Editable ones, defined via ` ```rust,editable `

Read-only code blocks support the usage of `#` to hide code from the view that's rendered in the browser:

You can click on the "Eye" toggle to show the hidden lines:

This makes it convenient to break an example across multiple code blocks without repeating content from the previous block (e.g., the definition of `UserId` in the example above).

Instructors often need to be able to edit code directly on the slide, but we lose the hiding feature if we mark the block as `editable`:

The `UserId` definition is shown, even if it was prefixed with `#` in the source.

# Solution

Modify the theme to introduce a new button, next to the "Copy" one, which allows instructors to switch from "Preview" to "Edit" mode.
In "Preview" mode, code is rendered as if the `editable` attribute had not been included.
In "Edit" mode, all hidden code is shown and the instructor is free to edit.

We won't support going back from "Edit" to "Preview" mode.