Explained when entries could not be deleted,added detailed error codes

There are two cases when entries cannot be deleted:
* If they are invoiced
* If they are part of an archived project

In either of these cases, we cannot delete the entry. We have updated
the API specification to make this clear.
Explain that expenses cannot be deleted if they have been invoiced

If an expense has been invoiced, then it cannot be deleted. We will
return a `422` response with a special error code that will incidate
that the expense has been invoiced.
Rewrite notice for invoice's delete action to include error code.

The notice about when an invoice cannot be deleted now fits the format
established in the rest of the application, and now includes an error
code that indicates that an invoice has been paid (and therefore
cannot be deleted).
Clarify that deleting a project group does not delete its projects

If a project group is deleted, its projects are not deleted as well.
Instead, they are simply removed from the project group before the
project group is deleted. This makes it so that those projects are not
associated with any project group.
Explain that when a tag is deleted entries are not deleted but updated

When a tag is deleted, we update the entries associated with that tag
so that the text of the tag is included as part of the entry's
description.
User Delete and Deactivate Actions now follow Delete/Archive pattern

The User Delete and Deactivate Actions are the same as the
Delete/Archive actions we have previously established, but we use a
different verb for Archiving when discussing users. To keep things
consistent between the application and the API, we have updated our
specification to indicate that these actions are the same, but use
different names.