Refactor notifications to prevent stacking.

Closes #3511, Closes #3512, Closes #3526
- show* methods now close existing passive notifications by
  default.  They also now take an optional options object where
  existing parameters such as "delayed" and "defaultErrorText"
  can be passed in as well as the new "doNotClosePassive" flag.
- Removed all explicit calls to notifications.closePassive except
  for the few places where it makes sense to call it separately.
Admin isn't shown "make owner" option

closes #3558
- Replaced canAssignRoles as "show user" conditional with new
"canMakeOwner"
Descriptive error if user by mail not found.

closes #3537
- Replaced generic NotFound error with descriptive NotFoundError.
Merge pull request #3561 from felixrieseberg/iss3558

Admin isn't shown "make owner" option
Merge pull request #3521 from jaswilli/notifications

Prevent stacking of notifications
Merge pull request #3560 from felixrieseberg/iss3537

Descriptive error if user by mail not found.