Mobile/note-sharing

[muhammetssen]

Closes #469

• Modified handleInitialLink function to support opening note pages via deep links

• Added share_plus library to make it easier to share notes

• Changed note class, added creator field, according to latest backend specifications

• Note that deep links only work in release mode, since in assetlinks.json file we are hosting, only the release key's hash is registered. You can try with flutter run --release command.

[bilalaytekin]

All features work nicely. Good job! Merging now...