Use minimum of tag count for tag cloud

The tag cloud minimum used to start at 0, then repeatedly compared to
the lowest count and chose the smallest, which is bound to be zero in
every case.

Discussed [in commit 8c26ea7c](https://github.com/TracksApp/tracks/commit/8c26ea7cb596c97e37213c0cc994e66ee5fd27b0#commitcomment-2719199).