Disable JOSM tab on key and tag pages.

Superseded by the new Project tab.
Show JOSM-related API calls as deprecated.
Set number of projects using a tag in top_tags table.
Make it easier to get at version of taginfo running on a site.
Make chart in overview tab on key page smaller if page is small.

This fixes #60. If the chart gets to small it doesn't look good any more,
because labels will overlap. But it is better than before.