Include line items tax amount in total computation

This pull request (PR) aims to resolve the following issues pertaining to the total_amount field on the contribution edit screen:

1. The displayed line item total amount does not currently include line item tax amounts on the edit screen. Refer to the screenshot below:

With this PR, the line item total amount now includes the line item tax amounts

2. In cases where the contribution's Financial type has associated tax, this tax amount is erroneously included in the calculation of the contribution line item total amount.

With this PR, the financial type at the contribution level has no effect on the line item total amount

3. The total amount of the contribution is directly editable as previously reported here: #43,

With this PR, the contribution's total amount will be disabled if there is more than one line item associated with it.