Create more statistics

[GHLasse]

Would be nice to create a few more statistics. Here are some ideas:

• [x] Number of currently available printers (Pie Chart)
• [x] Total number of prints per month over the last years (Area Chart)
• [x] Total number of prints per month comparing the past three years (Line Chart)
• [ ] Total number of Workshop and Online Prints per year since 2014 (Stacked Column Chart)
• [x] Total number of unique customers per year since 2014 (Column Chart)
• [x] Ratio of Succesfull/Total number of prints per printer type (Bar or Column Chart)
• [x] Total number of prints this month (number displayed below available printers chart)
• [x] Total amount of material used in the last 12 month (number displayed below available printers chart)
• [x] Total number of unique users since creation (number displayed below available printers chart)

[GHLasse]

also created new themes for charts in that commit :-)

[GHLasse]

Charts don't display properly in divs in modal => maybe the instructions in https://github.com/ConsoleTVs/Charts/tree/5.4.0/docs/5#charts-in-tabs-cjarts-in-tabs can help?

[GHLasse]

Layout issues are fixed and all charts implemented. Only the total number of Workshop and Online Prints per year since 2014 is difficult, since stacked column charts aren't supported. I can get the numbers, but having the numbers as columns side by side does not result in a pleasant looking chart. If anyone has an idea for this or further charts, a new issue can be raised.