pass config to webview through new Observable-based webview API

[sqs]

The webview protocol message config had 2 problems:

1. It was sent (in ChatController.sendConfig) whenever ChatController thought it might have changed, which meant a bunch of unnecessary calls to sendConfig sprinkled throughout ChatController.
2. It is a different schema from the other configuration, with configFeatures and other things having custom logic to construct them in ChatController.

This solves problem 1. Now, the config is sent through the legacyConfig(): LegacyWebviewConfig observable webview API method to the webview, which means it gets an up-to-date view of the config. It uses the same type, so this does not yet address problem 2, but it makes it much easier to address it.

Removes the old config webview protocol message.

There is no user-facing behavior change.

Test plan

CI and e2e

[dominiccooney]

Ad-hoc testing by @pkukielka indicates this regresses login redirection for free/pro users in JetBrains, and exacerbates a problem where theme styles sometimes do not apply.

Frankly, the rate of these changes is too high given the latency of the JetBrains QA and release cycle. We need authors to do more manual testing on JetBrains and contribute JetBrains tests if we want to keep producing releases.

[github-actions]

‼️ Hey @sourcegraph/cody-security, please review this PR carefully as it introduces the usage of an unsafe_ function or abuses PromptString.