Changes for external coupling of must-run interconnectors

[mabijkerk]

See quintel/etsource#3155. I made the following changes:

• Added FLH external coupling inputs to front-end

Notifying @kaskranenburgQ and @louispt1. There are a couple of to-do's still left before we can merge:

• Depending on the discussion about the capacity profile in Changes for external coupling of must-run interconnectors etsource#3155, we may need to update locales
• When uploading the must-run curve through the front-end, it says that an error occurred, after refreshing however, the curves are actually uploaded
• After adding the FLH external coupling inputs, the disabled availability and costs inputs are no longer hidden

[mabijkerk]

Given the limited time available from @louispt1, we're not going to address the second and third issue now. We should add them to #4345.

[kaskranenburgQ]

At the moment, the capacity slider is also hidden when the external coupling is set to active. This is because the capacity sliders are in the same interface_group (called 'capacity') as the interconnectivity sliders. This is specified in the file flexibility_electricity_import_export_interconnectivity.yml
There are at least 2 ways to solve this:

• Splitting the interface groups
• Adding an external coupling slider that sets the capacity (@mabijkerk we discussed this option already, could solve this as well)

[kaskranenburgQ]

After adding the FLH external coupling inputs, the disabled availability and costs inputs are no longer hidden

After reloading the page a couple of times, this can occur and vanish again as well. Seems to be a bug in the front-end.

[mabijkerk]

Good feedback @kaskranenburgQ. About the interface groups: let's await what the client prefers and see if we can redesign the frontend afterwards. Any remaining bugs we can add to the existing issue and dev should pick them up when they have time.

[kaskranenburgQ]

When the external coupling external_model_interconnectors is set to 1.0, all the slide texts for the curves will be set to "Coupled custom curves". Even when the interconnector is not coupled to an external model, as seen here:

Ideally this would also depend on whether the interconnector in question is coupled.

[mabijkerk]

As stated in quintel/etsource#3155 (comment), we can close this PR because the FLH inputs are removed. We can then open a new PR to tackle the issue with the interface group and we still need to add the front-end issues to #4345.