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 b/src/static/img/undraw_insert.svg
new file mode 100644
index 0000000..1f5d4b2
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+++ b/src/static/img/undraw_insert.svg
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diff --git a/src/static/img/undraw_insert.svg.license b/src/static/img/undraw_insert.svg.license
new file mode 100644
index 0000000..fb1a091
--- /dev/null
+++ b/src/static/img/undraw_insert.svg.license
@@ -0,0 +1,7 @@
+undraw_insert.svg image obtained from https://undraw.co/. A copy of the license can be found below:
+
+Copyright 2022 Katerina Limpitsouni
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