Githawk: Migrate IssueLabeledCell to AttributedStringCell

[rnystrom]

See #864 for how-to

Requires adding custom drawing for rounded background colors on labels. See:

• https://stackoverflow.com/questions/21857408/how-to-set-nsstrings-background-cornerradius-on-ios7
• https://stackoverflow.com/questions/16362407/nsattributedstring-background-color-and-rounded-corners

[Sherlouk]

I might have a look into this if nobody else does, tried to do this effect before and it beat me 😂 round two!

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[Sherlouk]

Very much WIP, but this is what I av so far -- All one big attributed string!

[rnystrom]

Dang that looks awesome!!

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