Envelopes should be editable about swimo

[pwizla]

User story:
A user can click on an item from the 'Envelope' column in BudgetTable to modify the envelope amount.
Modification of the envelope should render real-time update of the other amounts on the same line in the table (engaged, remaining).

UI implementation suggestions:

• minimalistic: click on item -> edit -> onBlur event to save
• more elaborate approach: hovering over an item -> an edit icon appear -> click the icon or item to edit -> press Enter / Esc or click on save / cancel button (green check / red cross)

[pwizla]

Fixed in this commit

Chosen UI approach: the minimalistic one.