[fix bug 765783] Implement callback url support.
[fix bug 765783] Implement callback url support.

- For each successful subscription in /contribute we POST to
  callbackurl GET parameter if available.

- Added /contribute/event/ page that renders the same page as
  /contribute but returns user to the same page after successfull
  subscription, so a new user can directly join.

chirilo/bedrock,ckprice/bedrock,pascalchevrel/bedrock,elin-moco/bedrock,CSCI-462-01-2017/bedrock,mermi/bedrock,amjadm61/bedrock,mahinthjoe/bedrock,jpetto/bedrock,davehunt/bedrock,SujaySKumar/bedrock,kyoshino/bedrock,mozilla/mwc,amjadm61/bedrock,gauthierm/bedrock,mmmavis/lightbeam-bedrock-website,MichaelKohler/bedrock,sgarrity/bedrock,l-hedgehog/bedrock,flodolo/bedrock,ckprice/bedrock,MichaelKohler/bedrock,davidwboswell/documentation_autoresponse,rishiloyola/bedrock,mmmavis/bedrock,sgarrity/bedrock,chirilo/bedrock,jgmize/bedrock,CSCI-462-01-2017/bedrock,mozilla/bedrock,alexgibson/bedrock,rishiloyola/bedrock,TheJJ100100/bedrock,TheJJ100100/bedrock,TheoChevalier/bedrock,gerv/bedrock,alexgibson/bedrock,davidwboswell/documentation_autoresponse,mkmelin/bedrock,Jobava/bedrock,analytics-pros/mozilla-bedrock,ericawright/bedrock,jgmize/bedrock,hoosteeno/bedrock,mermi/bedrock,pascalchevrel/bedrock,Sancus/bedrock,amjadm61/bedrock,malena/bedrock,mozilla/mwc,bensternthal/bedrock,glogiotatidis/bedrock,mahinthjoe/bedrock,flodolo/bedrock,mahinthjoe/bedrock,elin-moco/bedrock,TheoChevalier/bedrock,CSCI-462-01-2017/bedrock,hoosteeno/bedrock,SujaySKumar/bedrock,sylvestre/bedrock,marcoscaceres/bedrock,mkmelin/bedrock,jpetto/bedrock,jacshfr/mozilla-bedrock,pmclanahan/bedrock,bensternthal/bedrock,petabyte/bedrock,yglazko/bedrock,malena/bedrock,craigcook/bedrock,jacshfr/mozilla-bedrock,analytics-pros/mozilla-bedrock,chirilo/bedrock,malena/bedrock,flodolo/bedrock,mmmavis/lightbeam-bedrock-website,marcoscaceres/bedrock,amjadm61/bedrock,mahinthjoe/bedrock,dudepare/bedrock,analytics-pros/mozilla-bedrock,petabyte/bedrock,MichaelKohler/bedrock,flodolo/bedrock,alexgibson/bedrock,rishiloyola/bedrock,sgarrity/bedrock,gerv/bedrock,ericawright/bedrock,hoosteeno/bedrock,petabyte/bedrock,mozilla/bedrock,schalkneethling/bedrock,mmmavis/lightbeam-bedrock-website,davehunt/bedrock,Jobava/bedrock,andreadelrio/bedrock,gauthierm/bedrock,chirilo/bedrock,SujaySKumar/bedrock,pmclanahan/bedrock,malena/bedrock,Jobava/bedrock,sgarrity/bedrock,mkmelin/bedrock,l-hedgehog/bedrock,kyoshino/bedrock,schalkneethling/bedrock,dudepare/bedrock,sylvestre/bedrock,SujaySKumar/bedrock,gerv/bedrock,mmmavis/bedrock,Sancus/bedrock,CSCI-462-01-2017/bedrock,andreadelrio/bedrock,jpetto/bedrock,dudepare/bedrock,sylvestre/bedrock,mmmavis/bedrock,rishiloyola/bedrock,glogiotatidis/bedrock,kyoshino/bedrock,mkmelin/bedrock,mozilla/mwc,yglazko/bedrock,Sancus/bedrock,mermi/bedrock,amjadm61/bedrock,dudepare/bedrock,schalkneethling/bedrock,davidwboswell/documentation_autoresponse,ericawright/bedrock,kyoshino/bedrock,mozilla/mwc,marcoscaceres/bedrock,Jobava/bedrock,TheoChevalier/bedrock,mmmavis/bedrock,gauthierm/bedrock,alexgibson/bedrock,pascalchevrel/bedrock,ckprice/bedrock,schalkneethling/bedrock,pmclanahan/bedrock,hoosteeno/bedrock,davehunt/bedrock,TheoChevalier/bedrock,elin-moco/bedrock,ericawright/bedrock,yglazko/bedrock,analytics-pros/mozilla-bedrock,elin-moco/bedrock,gerv/bedrock,petabyte/bedrock,andreadelrio/bedrock,pascalchevrel/bedrock,jacshfr/mozilla-bedrock,glogiotatidis/bedrock,jgmize/bedrock,Sancus/bedrock,glogiotatidis/bedrock,bensternthal/bedrock,mozilla/bedrock,sylvestre/bedrock,craigcook/bedrock,craigcook/bedrock,gauthierm/bedrock,marcoscaceres/bedrock,davidwboswell/documentation_autoresponse,mozilla/bedrock,jacshfr/mozilla-bedrock,pmclanahan/bedrock,l-hedgehog/bedrock,davehunt/bedrock,ckprice/bedrock,MichaelKohler/bedrock,bensternthal/bedrock,craigcook/bedrock,TheJJ100100/bedrock,yglazko/bedrock,mermi/bedrock,l-hedgehog/bedrock,jacshfr/mozilla-bedrock,jpetto/bedrock,jgmize/bedrock,TheJJ100100/bedrock,andreadelrio/bedrock