Stabilize code of Accounts services: finish implementation and write tests

[proshin-roman]

No description provided.

[0pdd]

@proshin-roman 3 puzzles #69, #70, #71 are still not solved; solved: #unknown, #unknown, #unknown.

[0pdd]

@proshin-roman 5 puzzles #108, #109, #69, #70, #71 are still not solved; solved: #unknown, #unknown, #unknown.

[0pdd]

@proshin-roman 4 puzzles #108, #109, #70, #71 are still not solved; solved: #69, #unknown, #unknown, #unknown.

[0pdd]

@proshin-roman 3 puzzles #108, #109, #70 are still not solved; solved: #69, #71, #unknown, #unknown, #unknown.

[0pdd]

@proshin-roman 2 puzzles #108, #70 are still not solved; solved: #109, #69, #71, #unknown, #unknown, #unknown.