Support of importing data from other apps

[EmmaTellblom]

It would be easier for users to switch from other apps to Mensinator if we could import that data.

We should look at supporting import from the biggest apps such as:
Flo
Clue
Ovia

Actions:
Get the export formats from these apps.
See if we can determine type of import automatically or if we need to have the user define the app they are moving from.
Extend the import functionality.