Implement the dashboard data export

[antoan2]

On a click on the "Exporter les données" on the dashboard app, we need to fill 3 sheets of an excel document (one for each dataset (osfi / odrive / chorus)) :
Some resources :

• Dash download menus
• Pandas excelwritter

[Tiyop]

WIP on 'export-feature' branch. Need to:

1. Create util function to generate Excel file as bytes array
2. Create route + callback for user to download data

[antoan2]

About the util, it might be easier to just have an excel template saved, and just copy it.
And the excel could be already filled with some functions on the main sheet.
What do you think?