Hide unknown variants by default and fix EVS allele freq error

When filtering genes by letter, unknown variants are now hidden by
default. A divide by zero error caused when an EVS allele
frequency was missing has now been fixed.
Remove 'Data' and 'Pathogenicity assessment' sections from help page
Modify loading icon to show from any page

The loading icon now shows from any page when you try to filter genes by
letter. The button text to show/hide 'Unknown significance' variants was
also modified slightly.
Fix minor bug with loading icon display