Add first variant of schemas and migrations

[DecafMangoITMO]

Для запуска:

1. Поменять данные доступа к БД в файле config/dev.exs
2. mix ecto.create
   mix ecto.migrate
   mix ecto.drop (чтобы все сбросить)

Основная идея:

1. Схемы (а-ля описания сущностей) лежат в lib/backend/schemas
2. Миграции лежат в priv/repo/migrations

ATTENTION!
Для создания новой миграции следует использовать mix ecto.gen.migration <название>

[worthant]

• UPD: Fixed linter warnings
• Note: need to fix ci/cd testing errors. Pr not approved for now.

[coveralls]

Pull Request Test Coverage Report for Build a9cb91312e45a5082f40c6866949b4eb53558b5f-PR-2

Details

• 0 of 2 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 26.087%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/backend/schemas/file.ex	0	1	0.0%
lib/backend/schemas/user.ex	0	1	0.0%

Totals
Change from base Build 886b626480c0c8f3ad0a592fa965c2e68e17863c:	-0.3%
Covered Lines:	48
Relevant Lines:	184

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💛 - Coveralls