Autofilter

.. let the user choose metadata texts to filter
   by value in the sidebar.

Note there are issues;

1. the value lists are not updated automatically when
   rides are updated or imported and values are added
   or removed.

2. it only works with shorttext metadata fields, we might
   choose to add numeric etc later.

3. it would be nice to get an indication of how many
   activities contain the value and maybe even sort
   by count.