Add quotes to value of date key for DiskUsage plugin in webviz-full-demo.yml

[sigurdp]

The YAML parser currently used in webviz-config, PyYAML, has a lot of built-in magic when parsing date values. This magic causes the value of the date key for the DiskUsage plugin in webviz-full-demo.yml to be interpreted incorrectly.

webviz-subsurface-testdata/webviz_examples/webviz-full-demo.yml

Lines 28 to 32 in 8520e3c

	- title: Disk usage
	content:
	- DiskUsage:
	scratch_dir: ../.
	date: 2020-07-11 # Date only necessary if you want an explicit date.

PyYAML ends up parsing 2020-07-11 as a datetime.date. This is not the expected input to the DiskUsage plugin which expects a string input. Until we either patch PyYaml or switch YAML parser, keys such as this should be enclosed in quotes.

Related to this issue in webviz-config: equinor/webviz-config#396