Can now access Article page again.

Using a CacheService to pass Article instances to the Article page
easily.
Moved from OnActivate to OnInitialize so that Articles are preserved.
MainView now shows progress indicator.

PivotItems also show a message like system apps.
Corrected Visibility binding.
Now catches errors when failing to get articles.

Using more UI to show and explain the errors to the user.
Removed all custom styling.

When I'm happy with the functionality I'll use the ThemeManager NuGet
package to set my styling as a custom theme.
Missed another style.
Now using BindableAppBar
Can now open Article in browser.
LatestView wasn't going to ArticlePage
ArticlePage now using SystemTray ProgressIndicator
Catching extra exception.

On a WiFi network that requires logging in through the browser, an
exception was being thrown as HTML was being returned from the response.
New LongListSelector

Added SelectedItem dependency property to subclass of LongListSelector
for easier binding.
Sets it to 'null' when navigating away so that the user can select same
item again. It did nothing before.
Added a trigger to fire when new items should be loaded for infinite
scrolling but it's not doing the actual loading yet.