Remove demographic additional variable and renamed existing evidence_…

[O-Mohammed]

…category variable to Study design

The evidence_category variable is either "study" or "review" which comes from the names of the sheets in the excel file. Figured it would be more useful to include these than to completely remove Study design option(?)

[O-Mohammed]

Country of study, Study design, effect and setting are all converted to factors before being put into the DT.

• The filter drop downs don't have much room to be able to read the categories
• Some variables have unexpectedly long values that aren't readable in the dropdown