Add tabs for JS vs React snippets, add snippet types

[corywatilo]

We only show the JS embed code in Squeak! settings, but we should have tabs with two code options:

1. React snippet (default)
2. Existing JS snippet

Variables should be automatically populated for both (like how we currently do for the vanilla JS code).

[corywatilo]

Cory to design this with different snippet types in mind:

• Ask only form
• Only asked on this page widget
• Widget that shows all questions (or limited by topic, or source, like Slack)
• Permalink snippet (single question ID)