UI for data upload

[nhoening]

We need a simple UI for data upload (think version 1 of something that can grow).

The use case here is a (potential) customer who wants to try FlexMeasures on their data for a simulation or for just seeing the forecasts.

They'll probably have a spreadsheet available, so that's what we'll support with this. We can make certain demands in this version (e.g. we'll only load two columns ― datetime and value).

A stretch goal would be to show a spinner after the upload, which spins until the forecasting job for this data (if one is enough) is finished. Or an email if that is the case (for which we'd need a monitoring task).

[Flix6x]

A related CLI task to load data from a simple CSV file is introduced in #85.