Remove wrong abstraction in IAccount and use impl instead

[ChristophWurst]

This removes a minor flaw in our OOP design. Those two methods are only implemented for a specific account, hence they shouldn't be part of the interface. In fact, the mail transmission logic even assumed to get an instance of Account and the impl in UnifiedAccount would throw an exception anyway.

Found while digging into the drafts code for #9.

[ChristophWurst]

FYI the Account class is by far the most complex class we have in our code base. It's tightly coupled to several other classes and does not leverage the advantages of IoC/DI and hence can only be tested partly (just look at the constructor and see how dependencies are located statically). Still it is one of the most used classes spread over the full code base. As a long time goal we should try to break it into functional blocks (something like the mail transmission service that I created).

[ChristophWurst]

Minor change, CI is happy and no feedback within a week -> merging.

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