Displayed numeric list shortcut conflicts with Open File shortcut in Safari

Pressing the indicated Cmd+O shortcut for creating a numbered list results in the Open File dialog being shown in Safari. In Chrome and Firefox the numbered list is created as expected.

**Steps to reproduce:**

1. Create a new annotation in Safari and focus the markdown editor
2. Press Cmd+O to create a numbered list

**Expected:** - A numbered list is created
**Actual:** - In Chrome and Firefox a numbered list is created. In Firefox the "File" menu item briefly highlights as if the Open File action was going to trigger, but it doesn't. In Safari the browser's Open File dialog appears and no numbered list is created.

**Notes:**

Our editor also accepts Ctrl+O as an alternate shortcut, which does work in Safari, as does Cmd+Shift+O.

Shortcuts for this action in some popular editors on macOS:

Stack Overflow: Ctrl+O
GitHub, Google Docs, Trello: Cmd+Shift+7 (and Cmd+Shift+8 for a bullet list)
Microsoft Word: Cmd+Slash (/)
Evernote: Cmd+Shift+O