Indicate if data shown in a tile is very old

[zestyping]

As background, please read:

• The wiki page "Profiles" for details on the layout of the CSV file and the chart screen.

The tiles shown at the top of the patient chart are defined in the profile, and the HTML for their display is generated in chart.html. Right now, each tile shows the latest observation for its concept. However, the observation could be arbitrarily old, and this would be misleading to medics if, say, the patient's pulse is from last week.

To solve this, we need to:

• Provide the chart Pebble template with access to the timestamp of the tile observations

• Configure the length of time considered "old" for each tile concept

• Apply a CSS class to the tile when the observation is considered old

• Add some text or an indicator to the tile when the observation is considered old