Genome accessibility page - add intro text and consistify legend

[alimanfoo]

The genome accessibility page needs some short intro text. Also the colour legend for the genotypes page is positioned with the intro text, but the colour legend for the genome accessibility page is positioned within the controls. Proposed to add text and move the colour legends:

""" Use this page to browse properties of the sequence alignments and variant call sets against the reference genome. Variant types: [orange] SNP [blue] Indel Genomic regions: [yellow] Repeat [red] Hypervariable [green] Core [black] Centromere """

Also note slight change to legend for genomic regions.

[alimanfoo]

Also one other very minor thing about genome accessibility page: s/Call type/Calling method/.

[pvaut]

Implemented