Consolidate extension side webview code

[RMacfarlane]

Issue Type: Bug

The PullRequestOverview class already extends IssueOverview, but PullRequestViewProvider now also shares a lot of code in common with it. Should extract and reuse common pieces.

Extension version: 2020.9.34023
VS Code version: Code - Insiders 1.50.0-insider (3425c9207bd8e28eb8f10272637c957a1177a15f, 2020-09-24T17:00:58.297Z)
OS version: Darwin x64 18.7.0