Implement Gitlab-Flow

[DmitriKonnovNN]

I suggest reconsidering original implementation of Git-Flow for the sake of Gitlab-Flow because this will prevent us from being overwhelmed by branching hell and on the flip side it'll give us an opportunity to have and deploy 2 environments.

Gitlab Flow

• main is our dev-branch
• all features and bugfixes (except for hotfixes which we won't face soon) are to be pushed into MAIN
• main branch is ought to have STAGING branch;
• staging branch is ought to have PROD branch;
• all the changes merely flow downstream, so it makes sure that all changes are tested in all environments;
• upstream-principle: should we find a bug in one of the environments, we'll create a feature-branch of main and cherry-pick changes to all environments.

Or, maybe we could make use of Github-flow with only main branch, where changes are deployed sequentially into different environments.