Prevent 2x token approvals when requesting additional approval

[rozzaswap]

Describe the bug
Initial token approval if the user has a partial quantity allowance with the token for the bridge currently takes 2 transactions, one for 0 and another for the desired amount. Related to #110

Fix:

The bridge should figure out how large the allowance should be and request a transaction for that amount. The user is free to modify that on the wallet.