Remove React.js from starting thread flow

[rafalp]

Part of the #1674 epic

Requirements

Shared

• Thread title
• Thread message
• Preview message
• Set watching preference for thread
• Attach files to post
• Plugins can extend form
• Plugins can extend validation

New thread

• Select category to post in
• Post with poll
• Post with moderation settings
• Plugins can vote if new thread should require approval

New private thread

• Enter participants