Add ability to save and view a list of workflows

This CL introduces the following functionality:

- Introduces some logic to handle the persistence of workflows in the
  browser storage.
- Lets the user save created workflows in the "create workflow" dialog.
- Shows a list of workflows. If the user hasn't added any workflow, a
  placeholder image is shown alongside a text which invites the user to
  create a new workflow.

Bug: twpowertools:74

Change-Id: Icba09d20468bafc1415b802a3c935e22669546e6
diff --git a/src/static/img/undraw_insert.svg.license b/src/static/img/undraw_insert.svg.license
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