Disease-ontology link fix

From issue #3025

external links was allowing for partial matches and 'do' would match to 'MODEncodeTissueEMB_dopa' and link to 'http://modencode.oicr.on.ca/fgb2/gbrowse/worm/?label=EMBRYO_FAX_WIG/EMB_dopa' instead of 'disease-ontology.org'

Constrained regex and added test

link for testing: /search/disease/heart