Turn tag soup into HTML5

Up to now AnkiDroid has not been using a doctype type, so what was displayed was basically a tag soup.
Some time ago @dae changed the display to HTML5 in Anki desktop, dae/old-ankiqt@567a3c6e019281bbbed28755da908075f4c8e729

This does the same for AnkiDroid, but also adds the required `title` tag.

Apparently the rendering of tag soups works differently than the rendering of HTML5.
I had to/could simplify the HTML to bring it in line with Anki desktop, i.e. remove the extra span and the table/table-element display.

N.B. With this commit, the vertical centering doesn't work any more.
But then again, that makes AnkiDroid more closely resemble the desktop version.

The changes in detail:

* Clean up whitespace and js indent style.
* Bring display in line with Anki desktop by removing the extra <span> and table+table-cell display.
* Add <!DOCTYPE html> to make the flashcard html5, as Anki desktop does.
* Add the required <title> tag and a content/encoding meta tag.
* renamed `flashcard_css` to `flashcard.css`