Suggest next useful metric when adding a new metric

[Sebastiaan127001]

Currently, when adding a new metric, Quality Time automatically 'suggests' the metric 'accessibility violations'. It would be useful if Quality Time chooses the new metric type based on the following criteria.

• Pick a metric from a list of metrics suitable for this subject type
• Pick a metric that is not already used
• (optionally leave out the less obvious metrics like: all the manual ones, arbitrary ones like team happiness, in short use only the ones that can be automated from a tool source)
• when QT knows which sources are available, this can also be taken into account ( see https://github.com/ICTU/quality-time/issues/3127 )

[Sebastiaan127001]

relates to https://github.com/ICTU/quality-time/issues/3130