Improve photo browser caption text.

* Decrease size of title.
* Increase size of caption.
* Use brand fonts.

https://app.asana.com/0/21460959388827/18997455830385
Add fallback fonts to photo browser caption

* Use standard system font should brand fonts be unavailable.
* Extract brand colour (Slate1) to a variable for reuse/self documentation.
* Remove some commented-out code.

https://app.asana.com/0/21460959388827/18997455830385
Merge pull request #3 from fanpix/18997455830385_photo_browser_text_update

Improve photo browser caption text.