Third try to do html5.

Up to now AnkiDroid has not been using any `doctype`, 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, so AnkiDroid should probably do the same.

I had done something like this with pull request #42. This is an updated version, incorporating the suggestions made there.

Changes:
* Rename the card templates.
* Don’t do horizontal align.
* Let the webkit worry about resizing images.
* Vertical centering without tables as an option. Thanks to @hssm
* Use inner div with margins and make margins smaller.
* Checkbox vertical centering (old style) or not (everything at the top, like desktop).
* Semi-automatically remove a few trailing spaces.
* The `font-size: 250%;`  i mentioned in the old pull request is still gone.
* So is the DIY resize code. I guess that old code may have had something to do with this [resize problem](https://groups.google.com/d/topic/anki-android/nATGqTRCKlc/discussion).
* The new version also adds the classes `"mobile android linux js"` to the `html` tag, as discussed in the first try.

This squashes all commits from the [second try](https://github.com/ospalh/Anki-Android/tree/v2.1-dev-html5-2-vcenterpref) into one. (I tried to keep the removal of the trailing whitespace separate, but it seems didn’t work. I hope that is OK.)