Adding support for NSAttributedString in WPNoResultsView message

[etoledom]

This PR adds support for attributes in the WPNoResultsView's message 'label'. This is used to show a tappable link in the Stock Photos feature in the Main WPiOS app.

These changes where already reviewed and accepted here, but since we were working in a feature branch, this changes didn't make it to develop on time.

This is a re-implementation of those changes so they are accessible in the feature branch after updating with develop in the main WPiOS project.

[jleandroperez]

Update Mark II:

(Suggestions!!)

1. Construct the string with the target Font + Color + Alignment
2. Then linkify all the things with a snippet like this:

extension NSMutableAttributedString {

    private var foundationString: NSString {
        return string as NSString
    }

    func linkify(anchorText: String, link: String) {
        var range = foundationString.range(of: anchorText)
        while range.location != NSNotFound {
            addAttribute(.link, value: link, range: range)

            let minimumRange = NSRange(location: range.location + 1, length: foundationString.length - range.location - 1)
            range = foundationString.range(of: anchorText, options: .caseInsensitive, range: minimumRange)
        }
    }
}

let test = NSMutableAttributedString(string: "Automattic Company Testing", attributes: nil)
test.linkify(anchorText: "Company", link: "http://www.a8c.com")

--

Squirrel still stands, this can be improved whenever the WPNoResults replacement is in place!

[etoledom]

We decided to add those improvements to the new NoResultsViewController

Meanwhile, I added a comment to the attributedMessageText property explaining intention and behavior.

Thank you @jleandroperez !