Implement local storage for Repository service

[lonely-lockley]

Repository service should be able to work with a single git directory for all projects and files.
This will be useful for single-user mode and will be improved after we get authentication.

[lonely-lockley]

Repository service should allow:

• creating a project - a directory in a workspace
• creating a file in a project
• list files in a workspace
• parse files headers to detect their level
• generate and keep file identifiers not to expose direct file system operations via API for security reasons

[lonely-lockley]

The idea behind a local storage is making two database tables: the first one is a repository. It may be local or point to a github project.
Each entry in the table must contain a unique id, owner id, a link to github (for remote project that will be added later) and a JSON field for project directory structure.
The second table must contain file entries: id, name, file content (blob), file version for git.

[lonely-lockley]

Closed by #56