(Chore) Properly handle numbers using BigNumber library

[fernandomg]

I'm creating this issue as a consequence of several bugs being raised due to the numbers handling across the wizard.

The idea is to:

1. eliminate duplicated operations,
2. cast all numeric values to be stored in contracts to String type,
3. cast all numeric values to be retrieved from contract to BigNumber/String,
4. move every calculated value to stores,

Steps to accomplish this task:

• Create a utility with all the necessary calculation
• Update stores to host values
• Refactor utils/utils.js
• Refactor manage/utils.js
• Refactor stepTwo/index.js
• Refactor stepThree/index.js
• Refactor stepFour/index.js
• Refactor stepFour/utils.js
• Refactor crowdsale/index.js
• Refactor crowdsale/utils.js
• Refactor invest/index.js
• Refactor stores/utils.js

A list of bugs related to this chore:

1. (Bug) Unprecise (fractional) supply value #708
2. (Bug) Use proper way to instantiate web3 objects #706
3. (Bug) Unprecise(fractional) rate value #694
4. (Bug) Unprecise value of total supply after modification of tier #674
5. (Bug) Unable to buy minimal (defined by decimal) value #671
6. Token wizard crowdsale creation does not call metamask #632

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Please, feel free to update this issue to add any necessary detail or checkpoint required to properly accomplish this task.

[fernandomg]

This issue served as a reference for the final-form migration of the whole wizard. Everything but ReservedTokens and WhitelistAddresses was migrated.