feat: user/device specific layouts

[matmen]

Creates user and device specific layouts on layout change (based on concatenation of dashboard-{user}-{device-size}, clones current dashboard if no custom one exists yet).

Allows the overriding of the default layout for new users (or other devices), which can also be reset to the initial state.
Not 100% sure about the placement and usage of header buttons here, but it's what was there before:

Closes #370
Closes #410

[pedrolamas]

Working fine, but I think that getter should be renamed to make clear it returns the layout name and not the actual layout objects!