Add ability to create group microcycle

[Gregory Smith]

On the Plan tab, the menu looks like

• Plans
• Sessions

• Plan sessions

  • Add session
  • Add group session
  • plan microcycle
  • clone multiple sessions

I think it would be logic to be arranged as follows

• Plans
• Sessions

• Plan sessions

  • Add session
  • plan microcycle
  • clone multiple sessions

• Plan Group Sessions

  • Add Group Session
  • Plan Group Microcycle
  • Clone sessions for group

[Sander Roosendaal]

fixing #482

→ <<cset e308c3e9f688>>