Fix bold newline issue in PublicLab.Editor

[Recxsmacx]

Fixes #658

Add a test to check for filtering out newlines with only formatting in test/ui-testing/bold.test.js.

• Add a new test case 'Filters out newlines with only formatting'.
• Switch to wysiwyg mode if it is in markdown mode.
• Click on bold button and add newlines with only formatting.
• Switch to markdown mode and check for filtering out newlines with only formatting.
• Reset changes by removing the added text and switch back to wysiwyg mode.

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