batches: revamp "create" page

Created by: courier-new

Closes #23534 (closed).

Here are what are hopefully some slight improvements to the presentation of information and flow of kicking off a batch change using SSBC:

https://user-images.githubusercontent.com/8942601/128438208-b70d8ee8-a1f7-4b75-aa4a-4d1c63aabb09.mov

I left the BatchChangeExecutionDetails page alone for this and just focused on some updates to the entry point based on our conversations around this screen design.

The main changes:

• The example spec files are editable, and save state per spec, so it has more of a "playground" feel. 🙂
• Distinguishes the two options for executing a batch spec, with the "how" front and center and the "why" not too far away
  • Side note: I need advice on what we want to recommend for why you'd want to run a spec in src-cli, and when you'd want to run it with SSBC! I added a couple points for src-cli here and for SSBC here but let's just say they aren't very serious 😅
  • Also looking for feedback on calling this process executing the batch spec "On Sourcegraph", since running it "server side" or using our other official internal lingo doesn't necessarily mean much to an outside user
• Takes the user directly to the execution page after you click "run", assuming it doesn't error out

I also did a bit of cleanup along the way, like converting the example spec tabs over to using @reach/tabs, dropping a broken link from the "minimal" spec file, and reorganizing the component hierarchy a tad.