Add option to add legend for individual codes

[milanwiedemann]

@HelenCEBM suggested the option to include a legend for individual codes in #97 when using the Show individual codes function in the Trends over time tab. Although this is difficult to implement without taking up too much space (e.g., a legend and colour scheme for 500 distinct codes wont fit on one page easily) we should consider other options because it would be very useful feature for sharing screenshots with colleagues. Here some options to consider:

• Add another switch next to Show individual codes that shows the top 5 codes with legend (e.g., Show top 5 codes with legend)
• Add an option to generate and download a report (html or pdf) for the selected codes, that includes a top 5 visualisation, this could for example be part of Add downloadable report for selected codes to Shiny app #114