add multiple boolean? to sidebarSelect attributes

[increddibelly]

Summary

This PR can be summarized in the following changelog entry:

• [schema-blocks] Fixes a bug where a sidebar select would not allow multiple selections.

Relevant technical choices:

• add multiple boolean field to the attribute definition and set default value

Test instructions

This PR can be tested by following these steps:

• we don't have any blocks yet with a sidebar select; this bug prevented the schema blocks package from being compiled. That problem is now gone.
• You could add a sidebarselect block to a recipe template, but the actual functionality of the sidebar select control has never been tested and is out of scope for this ticket.

Impact check

• This PR affects the following parts of the plugin, which may require extra testing:
  *

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #

[hansjovis]

CR: 👍

[hansjovis]

Acceptance test: ✅

• Package can be built.
• Tests pass.
• CS is okay.

Waiting for Travis to pass.