web: upgrade bootstrap to 3.2
web: support more than 2 postings in the add form

- ctrl plus/ctrl minus adds/removes posting fields
- clicking the more link or pressing tab in the last field also works
- the focus is adjusted sensibly
- the add form is reset if closed and reopened, except the number of postings
- keyboard shortcuts should be less dependent on focus now
- tested in recent firefox, chrome, safari
- things should be robust with typeahead, with one notable exception:
  typeahead is not enabled in the new account fields when you add
  postings. I tried hard, help welcome.