Improve loading times of the dash apps

[dalonsoa]

This PR addresses the slow response of the dash apps by doing the following:

• Reducing to a minimum the number of queries to the Measurement table, which is big and will only get bigger, storing the required information in a new variables field within the Station model.
• Prevent the initial loading of the data in the reporting app, rather requiring from the user to click a button, as it is done in the Validation app. This lets the user decide on the station, variables and range of interest before plotting anything.

[Sahil590]

It is better having the submit button and taking the dropdowns as state instead of as an input. Currently testing it with larger set of data but taking a while to load it in. LGTM from what I can see apart from the failing tests