workflows: add a root node

[oliver-sanders]

Add a node into the workflows sidebar to represent all workflows.

[awaiting example sketch]

The problem

It would be good to have a "root" node in the workflows sidebar:

• Provides an easy way to issue commands against all workflows (e.g. cylc stop, cylc trigger '*/*:failed', cylc broadcast ..., etc).
• Creates a top-level aggregation of task state totals (i.e. a failed job in any workflow will illuminate the red square in the root node).
• Creates a top-level aggregation of workflow warning triangles.

The solution

The data store tree has the following hierarchy:

• user
  • [workflow-part ...]
    • workflow

Currently we trim the "user" node out of the workflows sidebar, but if we left it in, that would do the trick.

However, we might want to customise the appearance of this node as:

• It might make it look like the UI supports multiple users (it might one day, but it doesn't at present).
• Displaying the user name here is superfluous.
• It might not be obvious that clicking the user allows controlling all workflows under that user.

E.g, we might want this node to read "all 23 workflows".

We might also want to hide this node if the workflows list is filtered in any way to prevent confusion (the "user" node represents all workflows, not all currently filtered workflows).

Closely related to #2351