Add generic support for "clickable" table cells

When a table cell contains a link, it sometimes makes sense to redirect
clicks on the table cell (but not on the link) to the link.

This was done on the commit summary cell in branch logs, using custom
code.  Extract that code and make it available for re-use, by setting
the "clickable" class on table cells.
Improve "Work log" usefulness on /showreview slightly

In addition to the name of the user, the timestamp, and the comment (or
rather usually "No comment",) include a summary of the changes in the
batch (lines reviewed, issues raised, and so on.)
Various /showbatch improvements

- Keep everything in one "pale yellow table" instead of one per category
  of listed comments ("raised issues", "written notes" and "replied to".)

- Include "resolved issues", "reopened issues", "converted into issues"
  and "converted into notes" as categories of listed comments.

  This covers all possible changes right now (I think) so /showbatch
  should no longer be an "empty" page in any case.

- Only include [display all] links for categories of comments that lists
  more than one comment.  Do include one for the top-level when there
  are sub-levels, though.

- Fix formatting of the "callout" sub-table containing lists of reviewed
  lines per file.  Misplaced "white-space: pre" interacted badly with
  indentation white-space inserted automatically in the HTML code.

- Some code cleanup.
Improve diagnostic when testing VM is already running

We didn't check, so went ahead and executed the

  VBoxManage snapshot $VM restore $SNAPSHOT

command, which promptly failed with ugly output because the VM wasn't
in a state where restoring a snapshot was allowed.