Improve iCal endpoint

[Minei3oat]

This PR adds two features to the iCal endpoint:

• use ctf id and pad domain as persistent UID
  • currently a UUID is generated for each request
  • the spec requires the UID to be persistent
• add ctftime_url as ATTACH
  • this allows to retrieve more information about the ctf from ctftime

[JJ-8]

Hi @Minei3oat, thank you for your contribution! I have not tested your code yet but I added one question to my review, please see above. Thanks!