New tab "External Defense". TODO:
- summary tab
- variation of round time (+- 5 mins)
Merge remote-tracking branch 'upstream/develop' into develop2

Conflicts:
	src/modules/model/habitat.jsm
Fixes:
- fight Preview in External defense tab was not working anymore (conflict with the fortress commit)
- slow defense computation was not working for fortresses
Merge pull request #5 from gmuller668/develop2

merge branch gmuller668