fix(webview): can't respond in comment thread in webview for MR/Issue

This is the culmination of 3MR effort to clean up the webview and use GraphQL for working with discussions. (previous MRs: !150 (merged), !152 (merged)).

This MR contains mainly Vue code changes that change the UI look and behaviour. There is only a small change to the extension itself (changing the send note endpoint).

I used the https://code.visualstudio.com/api/references/theme-color VS Code guide for colouring the components.

Known shortcoming

I wasn't able to implement meaningful state management so every time the textarea component stops being rendered user loses their input (collapsing, clicking cancel, or switching tabs in the editor). Implementing meaningful state management was not achievable in the scope of this issue.

Related to #257 (closed)