Some refacts

[guillemfondin]

• clean some code
• fix expo warning (rollback @react-native-picker/picker from 2.2.1 to 2.1.0, and up react-native to 0.64.3)
• Add .gitignore

[Melvynx]

Duplicate of https://github.com/francoismari/elyze/pull/5 ?

[guillemfondin]

In the idea sure, but I think most changes are differents