Fix a few theming issues

[donokuda]

There's still a handful of things to address still, but I wanted to get these in sooner than later.

I think one of the last things to fix here is making the dropdown triangle the same color as VsBrush.WindowKey. However, I want to use the same type of triangle seen in other places inside of Visual Studio:

---

Screenshots

[donokuda]

Pushed some changes to the "Create a review" view:

• Tighten up spacing between header + PR title
• Added a bit more spacing before the summary text field
• Fixed theming
• Change "Review summary" label to "Your review summary" (to be closer to the title of the view)

Before

After

[donokuda]

ok, I'm going to take a step back from this pull request for real 😛

One thing I'm stuck on and could use some guidance is how to change the expander arrow/triangle to the default version that (I assume) we get from Visual Studio:

I believe the ones we use here are the triangle down octicons

But I'd rather use the ones we see in the headers for each section (next to "Description," "Reviewers," etc)

By using the default expander triangle, we'll fit better with the Visual Studio UI brand. Also hoping it's less code to maintain in the long run 🤞

[grokys]

LGTM. I'm currently porting this branch to #1491 so will port these changes as well, and modify the Expander style to match SectionControl there too 👍