ModalDialogue for editing flex context

[nhoening]

The UI for the JSON field in #1266. It is based on the design of Editing Graphs in #1212.

Basically, here we represent each field in the schema in a modal dialogue. Each field can be undefined (null - and thus be sent in the API).

Most fields are of type VariableQuantityField, which can be a value or a sensor ID in our case. So representing that option is the novel innovation in this PR.

Here is the description of the UI:

• Each field in flex context gets its own card
• Each card needs its own info icon to explain the field
• the card for inflexible devices is the one most similar to editing graph cards, so start with that - it gets a few sensors set.
• All 5 fields aside from inflexible_device_sensors can be in one of three states (which we should model with a toggle button - I like the second version): "Not set", "Fixed value", "Dynamic value".
• Fixed value means a text field is shown for entering the fixed value (needs to be a number), plus a unit dropdown selector (with allowed units for that field).
• Dynamic value means we can select one sensor on the right. (so if we select another, it replaces the previously selected one)
• Each card has a set of valid units and if the user selects a sensor with the wrong unit they see a warning.
• On the right we need a sensor search (much like for sensors_to_show)
• inflexible device sensors is a special case: It requires a list of sensors. Actually, here much of the sensors_to_show logic can be applied directly.

[nhoening]

Let me know if I should create a visual mock-up