Fix/primary currency fiat insufficient funds error

[Fatxx]

Description

This PR adds the following changes to Amount screen:

• Inserted amount is now validated correctly when Fiat is selected as primary currency in Settings;
• It should now provide converted Fiat values to all tokens that have conversion rate available, not only for mainnet;
• Improve safety while converting number to BN;
• Fix inconsistent decimal cases between native tokens and ERC-20 tokens when converted to Fiat;

What to test

• Case 1 - Insufficient funds fix

  • Switch to "Binance Smart Chain Mainet"
  • Switch to "Fiat" as primary currency under "Settings > General"
  • Go to Send view
  • Send a value under your balance, it should proceed.

• Case 2 - Insufficient funds fix

  • Switch to "Binance Smart Chain Mainet"
  • Switch to "Fiat" as primary currency under "Settings > General"
  • Go to Send view
  • Send a value above your balance, it should error "Insufficient funds".

• Case 3 - Convert FIAT value for tokens (non-ETH)

  • Switch to "Binance Smart Chain Mainet" (other networks can also be tested)
  • Switch to "Fiat" as primary currency under "Settings > General"
  • Go to send view
  • Insert FIAT value
  • It should show converted BNB value

Note: To validate correct conversion values please compare it with cryptocompare

Screenshot

Checklist

• There is a related GitHub issue
• Tests are included if applicable
• Any added code is fully documented

Issue

Resolves #3983

[github-actions]

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