Provide singular and plural versions of the unit name

[fniessink]

Tasks:

• Add singular and plural version of units to the data model.
• The user should be able to override both the singular and the plural version of the unit name.
• Use the singular version of the unit when the metric value is 1.
• Use the overridden version of the unit name in the measurement entity table headers.

Not to be changed:

• The metric type as displayed as header when the metric is expanded.