Implement "Replace Results" bar

Created by: courier-new

Scope of this ticket is limited to basic UI form of the bar integrated into the search page, based on Figma designs.

• Feature-gated find-and-replace button
• Find and replace form fields appear underneath when it's activated
• "Find" input field has buttons for case sensitivity, regex, structural, and clipboard
• The button for structural mode is disabled and has "coming soon" label
• "Replace" form field has button for clipboard
• There is a "Preview Pull Requests" button that is disabled until both a "find" term and a "replace" term are provided
• There is help text that assures the user they'll get to preview changesets first. Link to "Show me how it works" is a separate issue.