Claim/Minting logic for the Contribution Tokens after the Community Round

[lrazovic]

Description

After the Community Round, a new Contribution Token is created with the metadata provided by the user. #9
This PR adds the logic for the claim_contribution_tokens() extrinsic, which calculate the amount of CTs to which a user is entitled, and mint this amount to the user.

Changes Made

• The frame_support::traits::tokens::fungibles::Create trait (which "inherits" the Inspect trait) now uses T::ProjectIdentifier and T::CurrencyBalance as types for the AssetId and the Balance.
• The value of the Contributions StorageDoubleMap now is a ContributionInfo<T> instead of BalanceOf<T>, to keep track if the user can claim the Contribution Tokens.
• A user can now contribute multiple times to the same Project

Additional Notes

• At the moment the number of decimals used is taken from the storage, it will be changed in the future to be agnostic and resistant to changing decimals. The feature of claiming CTs not yet claimed after a certain number of blocks will be implemented in a later stage.
• For now Assets and the Native Token share the same type, if in the future we want to use a different type, we can implement something similar to https://github.com/paritytech/substrate-dex/blob/922c0b877c950a050aa3c8ba08389dd3a34c1fe1/src/lib.rs#L81