fix: withdraw/deposit converts from BZZ (10^16 base units) (#66)

* refactor: added toBZZbaseUnit function

* feat: added utility for attesting value is BZZ convertible to base units

* fix: conversion from 15 to 16 decimal places, added unsafe versions

* refactor: withdraw modal uses the safe conversion from BZZ

* refactor: added BigNumber and Token class to handle BZZ digits correctly

* refactor: extract deposit and withdraw functionality into single modal

* test: added tests for Token

* chore: removed unused component

* chore: addressed PR review, token decimal is now integer 0-18

* chore: added comment to clarify the value restriction on token amount

package.json

@@ -28,6 +28,7 @@
     "@types/react-router": "^5.1.13",
     "@types/react-router-dom": "^5.1.7",
     "axios": "^0.21.1",
+    "bignumber.js": "^9.0.1",
     "feather-icons": "^4.28.0",
     "material-ui-dropzone": "^3.5.0",
     "qrcode.react": "^1.0.1",