expui 1.0.0.7: collapse links without href are keyboard buttons

exp::collapse: a link that is no control by itself (an <a> without href, a <span>) and has no role of its own gets role="button" and tabindex="0" (a tabindex of its own is kept), and Enter and Space toggle it.

The autosave preview's link (#preview-link) is such a link: it carried aria-expanded, which a plain <a> without href may not have (accessibility checks reported it as a serious finding), and it could not be reached with Tab. Both are fixed.

Tests: two new browser tests in collapse.test.js; the preview test checks the link's role and tab stop. The browser suite has 116 tests.