Add functionality to link transactions upon transaction creation

[letehaha]

Right now on UI users can link only already-existing transactions. Need to implement an ability to link upon its creation.

Case 1: User creates transfer transaction, wanna create expense and link to existing income.
Case 2: User creates transfer transaction, wanna create income and link to existing expense.

In both cases, the user should be able to link. Right now to do so, the user first needs to create an expense or income, and only then the user can link it with the existing opposite transaction.

The backend is ready, only the UI part requires changes (unless it's not 🥲 ).

This is an "unfinished" part of that PR: #270