Svelte: Make search header primary, with repo subheader

[taiyab]

Loom: https://www.loom.com/share/b8c9a5a91c454e30b7fdd0af5aa6e498

This is a reversion of some of the work we did in merging the repo and global header into one thing and hiding the search input.

I want us to maintain the search input at the top and keep it primarily accessible.

This is because:

• it's less of a jump for existing users and makes the UI more familiar
• it allows us to experiment with making that input a much more primary way to navigate around Sourcegraph
• search is the core, intrinsic feature of Code Search
• removing other visual noise from the page has bought us the ability to keep a core focal point in the UI without it feeling too busy (search)

There are a ton of other reasons too, like not having the header morph too much as you go from page to page in the app.

Designs: https://www.figma.com/design/EPgl0qVbqGSumvf52brhda/Code-Search?m=auto&node-id=1947%3A37160&t=8ZWFvMEg01LRgGvX-1

Prototype: https://www.figma.com/proto/EPgl0qVbqGSumvf52brhda/Code-Search?page-id=1397%3A21372&node-id=1947-37160&viewport=-829%2C340%2C1&t=0r4Jim8IedP2RQdW-1&scaling=min-zoom&starting-point-node-id=1947%3A37160&show-proto-sidebar=1