Themes: Sheet details view

[ehg]

We want to view a theme's general, support and documentation pages.

TODO:

• SectionNav that allows switching between pages Themes: Sheet action bar #3333
• Render HTML into content div Themes: Bootstrap theme-details store for theme sheets #3279 — heavily restrict tags, try to avoid setting HTML dangerously TBD
• Clean markup, do we need a new endpoint for this?

UI:

[ockham]

Render HTML into content div #3279 — heavily restrict tags, try to avoid setting HTML dangerously TBD

Maybe we can re-use code from Reader for that?

[seear]

Markup comes output-buffered from theme.wordpress.com blog, so should be trusted.