VUU-333: Include username in local storage layout key

[pling-scottlogic]

Description

When using the local implementation of layout management, the relevant information is saved to local storage under the keys layouts/layouts, layouts/metadata and api/vui. This can cause problems once a second user is introduced, so we change the keys here to include a username. We also add the username to cookies for the NewTheme showcase example, matching the behaviour of the sample app.

Change List

• Add username to local storage keys (application layout, layout and metadata)
• Convert private functions in LocalLayoutPersistenceManager to arrow functions
• Get actual username from cookies
• Add username to cookies for showcase example

Testing

Saving and loading layouts tested using local and remote implementations, from both showcase and sample app.

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