SkeetView: Render rich text
Parse and display rich text (links, mentions, tags) within a skeet's content. This is done by processing the `facets` data associated with a post.
The `SkeetData` model now includes `facets` and a new `annotatedContent()` method to build an `AnnotatedString` for rendering. The `SkeetView` uses this new method to display the formatted text.