Move posts/PM view loading screen to proper UIViews.

Continuing the process of moving as much as possible out of the UIWebView; this
time it's spin.js and showing any particular loading screen.
Switch Macinyos color in theme picker to an OS 9 purple.

I think it was one of the default backgrounds? Has a tiled embossed Finder icon.
Bump secondary thread tag up and left 1pt.

Looks nicer (via @ridilla). The -setTagBorderColor:width: method on
AwfulThreadTagView is necessary because thread cells would set the tag view's
layer's border, which draws over the secondary thread tag. Setting the border
of the tag's image view does what we want.