Allow users to add/show descriptions of workflow templates

[terrytangyuan]

If the annotation workflows.argoproj.io/description is available, present it nicely in the UI and allow users to edit it.

[jmeridth]

If someone adds the following:

apiVersion: argoproj.io/v1alpha1
kind: WorkflowTemplate
metadata:
  name: my-awesome-workflowtemplate
  annotations:
    workflows.argoproj.io/description: 'The most amazing workflowtemplate EVAR'

to a workflowtemplate it will be available in the UI via https://argo-workflows-url/workflow-templates underneath the workflowtemplate name.

[agilgur5]

Superseded by #12644