remove Cody "upsells"—and all Cody links if Cody is disabled

[sqs]

At a high level, we don't want to show annoying ads/upsells for Cody that are not useful. And if Cody is disabled, we don't want to show any links to Cody.

Detailed desired behavior

• Dotcom
  • Navbar
    • Unauthed: "Cody" single link to /cody (marketing page)
    • Authed: "Cody" dropdown with "Dashboard" (/cody/manage) and "Chat" (/cody/chat)
  • Routes
    • /cody: always the marketing page
    • /cody/manage: requires sign-in, shows Cody PLG subscription status for the user (Free plan is auto-opted-into by default)
    • /cody/chat: requires sign-in
• Enterprise with Cody enabled on instance
  • Navbar
    • Cody NOT enabled for current user: "Cody" single link to /cody/dashboard
    • Cody enabled for current user: "Cody" dropdown with "Dashboard" (/cody/manage) and "Chat" (/cody/chat)
  • Routes
    • /cody: this link should not be present anywhere, but redirect to /cody/dashboard
    • /cody/manage: informational page, with editor/web links for Cody-enabled users and a "contact admin to get access" message for Cody-disabled users
    • /cody/chat: chat for Cody-enabled users, redirect to /cody/manage for Cody-disabled users
• Enterprise with Cody NOT enabled on instance ("cody.enabled": false in site config)
  • Navbar: no Cody link or dropdown
  • Routes: all Cody routes 404
• All
  • Do not show a Cody upsell on the /search page

This is an example of what we will KEEP for users on instances with Cody enabled but who do not themselves yet have access to Cody. This is useful because it informs users how to get access to Cody, and presumably their site admin wants people to request it who want to use it.

Fixes https://linear.app/sourcegraph/issue/SRCH-529/hide-cody-ai-tab-and-cody-upsell-if-cody-is-not-enabled

Unexpected code changes needed

This ended up being a much bigger change than I expected because I found error-prone code that needed cleaning up:

• Improve how we determine if Cody is enabled in the frontend code. Previously, we checked the license features in some places, cody.enabled site config in others, and the user's current RBAC permissions for Cody in yet others. The most error-prone was checking the license features, since a license may entitle the instance to Cody but the site admin may still choose to disable it. There were no places in the frontend code where checking the license's entitlements was actually correct, so I changed everything to checking either window.context.codyEnabledOnInstance or window.context.codyEnabledForCurrentUser.
• Did the same for window.context.codeSearchEnabledOnInstance for symmetry.
• Removed "helper" functions that just checked 1 or 2 boolean values on window.context related to this, in favor of accessing window.context directly. Globals aren't great, and we should use React context or something similar, but now that the JSContext has the right fields (i.e., enabled instead of licensed), it's simpler and there is no need for helper functions.
• Removed prop drilling of the licenseFeatures that was unnecessary since these values are available in globals and were being set from globals at some arbitrary point in the React component hierarchy anyway.
• Updated the GlobalNavbar test snapshots.

Test plan

Run in 3 modes: (1) dotcom mode, (2) "cody.enabled": false in site config, (3) normal sg start.

Changelog

• When Cody is disabled in site config (with "cody.enabled": false), all links and UI elements about Cody are hidden from all users. Previously, when Cody was disabled, users would see some links informing them about Cody.