Session attendance control system

[mikron-ia]

Create a system to control session attendance.

The general idea:

• A session is created, using the existing Game system
• Once it is saved, the GM has an option to send an invitation - via mail or link (#148 - style)
• The invitation page allows to view the session data and confirm presence with varying degree of certainty (yes / likely / maybe / unlikely / no will suffice)
  • [optional] Default option in the message: "if you don't answer in [given time], you'll be assumed to have answered [option chosen at sending]"
  • [optional] Ability to add a note explaining things
• GM can afterwards mark presence as present / partial / absent
  • [for consideration] A null option, allowing the GM not to bother with this - but it should be for all, i.e. if you mark one, you have to mark all
  • This can be a separate task

[mikron-ia]

Some additional options are listed in #238; both tasks can be done together.