Improvements and fixes

Strings can now be added to many inverters or combiner boxes in one step instead of one device at a time.

• Strings can be added to several inverters or combiner boxes in one step, with all of a plant's inverters selected at once.
• Editing a single string now opens with that string's current values already filled in.
• If the connection to a data logger drops during its setup, "Reconnect" resumes it and keeps the step, credentials and value mapping.
• On plants without combiner boxes, adding a string no longer asks where to attach it.