Fix issues with employee records without slack user ids

[jessieay]

Right now, there are 100+ employee records on prod that do not have a slack user id.

This is problematic because we use the slack user id to send messages.

We should write code into our existing employee import tasks that:

• updates existing employee records that do not have slack user ids (find them by slack username).
• soft delete that employee record that no longer exist (if find by slack username returns nil)