Change DOCTYPE to HTML5 and make index.html conform.

Mainly involves:

* adding `alt` attributes to all `<img>` and `<input type="image">` elements, at least empty ones,
* adding `src` attributes to all `<img>` and `<input type="image">` elements. Uses a transparent image as a placeholder. blankcard.jpg is not needed; the card elements have backgrounds (which should be white, not off-white like name labels and speech bubbles),
* fixing duplicate `id`s (partly by deleting the old individual filter modal),

Other errors:

* `<button>` mustn't contain block elements
* boolean attributes, like `disabled`, do not take `true` and `false` as their values; they are true by existing, and if they have a value, it must be the same as the attribute name.
* `<label>` can only contain one labelable control.
* `<p>` can only contain inline elements and text.
* `<ul>` must contain `<li>` elements.
* `&` should be encoded as `&amp;`.

Side effect:
* Switching to HTML5 seems to add vertical space between lines of inline-block elements. The selection cards on the individual select screen were changed from `display: inline-block` to `float: left`.