Add Django management command to print corpus info

Added a Django management command that prints information about the
corpus to the screen in a tab separated format. This command can be
used to generate text files with information about the corpus.
Fix removing data from graph

The final series was not removed from the graph, although the graph
displayed the message 'no data available' after the last pair was
removed. This issue was fixed by adding a unique id to the chart.

A unique id should be added to all charts.

Also when adding new pairs to an emptied chart, the legend colors don't
match anymore. This should also be fixed.
Add scatter plot for corpus

Added a scatter plot that displays the percentages of posemo tags for
each text in the corpus. Next steps are to make the entity categories
displayed dynamic (currently it is hardcoded posemo) and to add scatter
plots for the genre data.
Also, the scatter data must be loaded automatically (currently, a button
has to be clicked)
Update corpus_info command

The corpus_info command was updated to show the details of the texts
selected for annotation.
Merge branch 'feature/corpus-print-info-command' into develop
Update corpus_info command to print original year of publication

The corpus_info command was updated to print the original year of publication.