Lead images!

- Remove screenOrientation hack from PageActivity, and resolve all the
  implications thereof.
- Modify the lead-section API request to retrieve the lead image
  automatically, along with the WikiData id (for future mischief). These
  things are now stored in the PageProperties object.
- Implement lead image in pages.
- Disable lead image in landscape mode for low resolutions.
- Don't show a lead image placeholder when the page doesn't have a lead
  image associated with it.
- Add dark gradient behind page title for better visibility, as well as a
  slight drop shadow on the text.
- In JavaScript, move any images in the lead section to the bottom of the
  section, so that the native lead image is followed immediately by text.
- In JavaScript, remove the page title from the lead section, and move the
  edit pencil to the right of the first paragraph.
- Make the Page object no longer Parcelable, lest someone try to put it
  into an Intent or savedInstanceState.

Things to look into:

- How does Picasso manage its image cache? Now that we're fetching much
  larger images, we should profile Picasso's filesystem usage when caching
  images, since we don't want it to grow too big.

Change-Id: Icec164de0d515a547d1647e06153a84e3b9bc088