Fix issue where selected text is completely obscured

[jcansdale]

On Visual Studio 2019, selecting markdown obscures it completely. This PR makes the selection appear with an opacity of .4 (which is the same as Visual Studio 2017).

What this PR does

• Don't highlight selected area of selection on InfoPanel
• Change opacity of selected markdown to .4 (for some reason the default is 1 on VS 2019)
• Move Markdown.xaml into GitHub.VisualStudio.UI assembly (it depends on Visual Studio brushes)
• Use static references to VsBrushes so we can be sure keys actually exist

Before and after

Text in info banners can no longer be selected. For example:

How to test

Info banners

1. Launch VS and click 'Clone or check out code' in Start Window
2. Click 'GitHub' under 'Browse a repository' and sign in
3. Choose a repo that has already been cloned
4. Attempt to select text on the info banner
5. Text should not appear selected

Markdown text

1. Open pull request details on GitHub pane
2. Expand Description node
3. Select some text of the description
4. Text should be highlighted with the text still visible (not redacted completely!)

Fixes #2354