Change CTA copy and add direct download link

Created by: vdavid

Context

We have CTAs on the "search" and "file" pages. We have three types right now: 1. "Sign up!", 2. "Download browser extensions!", 3. "Download IDE extensions!". We'll only deal with 2. and 3. in this issue.

Problem

We have two problems:

1. The current copy is not as good as we want it.
2. For BExt: Users have to visit a docs page first, then they can download the extension for their browser, which are two steps. One step would be better.

Solution

We have a Figma that shows the changes to be done.

1. Change the copy of the browser extension and IDE CTAs.
2. Change the BExt CTA to have two buttons rather than one. Determine the user's browser (Chrome, Firefox, or Safari), find the links to the corresponding Sourcegraph extension in all three stores, and link the one that matches the user’s browser.