move fixCorruptUser to the client

Conflicts:
	src/app/index.coffee
APIv2 init
challenges: add basic task stats for challenge subscribers
challenge: don't allow to create new challenge unless in group
APIv2 init2
WIP addTask in api.coffee
api_v2 bug fix
api_v2 bug fix
Revert "WIP addTask in api.coffee"

This reverts commit 81f2b6789ef66bd21f3fbaafa43ded19a02865b8.
tags: use explicit {#each users[_userId].tags} instead of model.ref private path {#each _user.tags}. fixed many tags issues. see https://github.com/codeparty/derby/issues/267
Merge branch 'develop' into challenges

Conflicts:
	views/app/filters.html
	views/app/tasks.html