LabelService: Read labels and label data from a config file in data repo

Currently, the name / colour of the labels under each label category is hard-coded in `LabelService`.

We can consider fetching this data from a config file (located in the data repository), as part of the app's session setup logic.
(the app should fetch the label data from a config file, before synchronising labels with the repo).

The label categories themselves (e.g. `severity`, `response`) need to be defined by the app (and not the config file), as several parts of our app expect the categories to exist. The config will simply allow the label names / colours to be customised easily by admin users. The config file will also be useful for features like #363 , where further label data is needed.

Note: we would need to validate the config file to ensure critical labels are defined (e.g. `duplicate` label).

Read LabelService and the app's session setup path first, then trace where label synchronization with the data repository occurs. Define the config shape and validation requirements from the issue, including critical labels such as duplicate; done means label names and colours come from the config while required categories and labels remain available to the app.

Written by the indexing model from the issue text.