fix: alert mail display invalid url (subpath in the SENTRY_URL_PREFIX was dropped)
fix: alert mail display invalid url (subpath in the SENTRY_URL_PREFIX was dropped)

for example::

    >>> settings.SENTRY_URL_PREFIX='http://example.com/sentry'
    >>> url='/team/project/1/'  #generated by django.core.urlresolvers.reverse
    >>> urljoin(settings.SENTRY_URL_PREFIX, url)
    'http://example.com/team/project/1/'
JamesMura/sentry,beeftornado/sentry,rdio/sentry,camilonova/sentry,BayanGroup/sentry,zenefits/sentry,TedaLIEz/sentry,wujuguang/sentry,kevinlondon/sentry,Natim/sentry,1tush/sentry,korealerts1/sentry,BuildingLink/sentry,jean/sentry,fotinakis/sentry,songyi199111/sentry,zenefits/sentry,drcapulet/sentry,mvaled/sentry,imankulov/sentry,gencer/sentry,nicholasserra/sentry,pauloschilling/sentry,kevinastone/sentry,ewdurbin/sentry,fotinakis/sentry,zenefits/sentry,jokey2k/sentry,wong2/sentry,1tush/sentry,korealerts1/sentry,fotinakis/sentry,JTCunning/sentry,gg7/sentry,ewdurbin/sentry,mvaled/sentry,looker/sentry,fuziontech/sentry,gg7/sentry,kevinlondon/sentry,jean/sentry,rdio/sentry,daevaorn/sentry,daevaorn/sentry,JamesMura/sentry,BuildingLink/sentry,ifduyue/sentry,zenefits/sentry,imankulov/sentry,wong2/sentry,nicholasserra/sentry,boneyao/sentry,BuildingLink/sentry,looker/sentry,mitsuhiko/sentry,BayanGroup/sentry,felixbuenemann/sentry,drcapulet/sentry,mvaled/sentry,argonemyth/sentry,nicholasserra/sentry,fuziontech/sentry,jokey2k/sentry,gg7/sentry,gencer/sentry,alexm92/sentry,mvaled/sentry,korealerts1/sentry,rdio/sentry,songyi199111/sentry,hongliang5623/sentry,ifduyue/sentry,argonemyth/sentry,kevinastone/sentry,BuildingLink/sentry,fotinakis/sentry,looker/sentry,wujuguang/sentry,gencer/sentry,ngonzalvez/sentry,gencer/sentry,mvaled/sentry,JamesMura/sentry,mitsuhiko/sentry,hongliang5623/sentry,wong2/sentry,songyi199111/sentry,wujuguang/sentry,ewdurbin/sentry,felixbuenemann/sentry,hongliang5623/sentry,ifduyue/sentry,Natim/sentry,TedaLIEz/sentry,camilonova/sentry,daevaorn/sentry,JackDanger/sentry,llonchj/sentry,vperron/sentry,JackDanger/sentry,mvaled/sentry,daevaorn/sentry,kevinlondon/sentry,alexm92/sentry,alexm92/sentry,boneyao/sentry,jean/sentry,llonchj/sentry,zenefits/sentry,fuziontech/sentry,looker/sentry,Kryz/sentry,ngonzalvez/sentry,JamesMura/sentry,beeftornado/sentry,camilonova/sentry,vperron/sentry,ngonzalvez/sentry,BuildingLink/sentry,beeftornado/sentry,gencer/sentry,BayanGroup/sentry,felixbuenemann/sentry,ifduyue/sentry,jean/sentry,Kryz/sentry,jean/sentry,imankulov/sentry,pauloschilling/sentry,pauloschilling/sentry,kevinastone/sentry,ifduyue/sentry,rdio/sentry,Natim/sentry,Kryz/sentry,TedaLIEz/sentry,jokey2k/sentry,JTCunning/sentry,JTCunning/sentry,argonemyth/sentry,1tush/sentry,llonchj/sentry,drcapulet/sentry,boneyao/sentry,vperron/sentry,JamesMura/sentry,JackDanger/sentry,looker/sentry