Evaluate postediting time and Levinsthein distance

Postediting time is recorded within translate5 since years. Make it available as statistic in the GUI and via API. What visualization makes sense is secondary in the first step. It can be simply a table with different numbers.

• Show it as column in the editor (hidden by default)
• Calculate the average by task
• Calculate the average by the current filtering in the task management grid.
• If filtered by "advanced filters" (third dimension of things that correlate with the task user assignment (job), then also the statistic is only calculated by those jobs in the filtering
• Group the average in the visualized statistic by match rate range and language resource and also by language combination and combinations of those. Only those 3 dimensions (match rate range, language resource and language combination) are shown in the statistic.

 Make configurable, if and above what postediting time segments are not used for average calculation.

Since the match rate is saved for every segment, make the match ranges used for the statistic calculation also configurable.

Summon up the time and the levinsthein distance for all editings of one segment by the same taskuserassoc