Create VSCE specific signup page

Created by: jjinnii

We have multiple re-directs from the VSCE to sourcegraph.com to promote account creation, however, we are directing users to Sourcegraph's generic signup page, where the messaging is not effectively communicating the value of account creation to a VSCE user. (For example, "Navigate through code with IDE-like go to definition and reference hovers" seems out of place when a user is coming as a VS Code user.)

It will be very powerful to have a signup page with copy that better communicates the value of account creation to an IDE-user.