Show scrollbars on webview. Also, moved webview config back to XML.

[CDRussell]

Asana Issue URL: https://app.asana.com/0/414730916066338/547397654936878
GitHub Issue: #170

Description

Add scrollbars to the webview.

Steps to Test this PR:

1. Visit arstechnica.com to ensure scrollbar visible against black themed webpage
2. Visit medium.com to ensure visible against white theme webpage
3. Ensure scroll bar only visible when scrolling, and fades away when not scrolling

[subsymbolic]

Nice, I like having scrollbars!

A few things to fix (they're probably all related):
• The omnibar no longer hides on scroll
• The bottom overscroll view no longer shows properly
• The scrollbar falls off the bottom of the page

Can we also make the scrollbars a dark gray color? When the bars are really large (as they are on a page that doesn't need to scroll much) the orange is loud.

[CDRussell]

The omnibar no longer hides on scroll

Switched out to a normal WebView during implementation and failed to switch back; 🤦‍♂️ . Fixed.

The bottom overscroll view no longer shows properly

Should work now.

The scrollbar falls off the bottom of the page

It shouldn't now.

Can we also make the scrollbars a dark gray color?

I've removed the accent color from the "thumb" and used a grey instead. It isn't dark, as the scrollbar "track" is already dark grey and we need a bit of contrast. Happy to tweak colours though, see colors.xml for their definitions if you want to experiment

<color name="webViewScrollbarTrackColor">#FF333333</color>
<color name="webViewScrollbarThumbColor">#FFABABAB</color>

[subsymbolic]

Looks great!