Implement "Replace Results" bar

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.

sourcegraph / sourcegraph-public-snapshot