Expand naming convention for commits, branches, and PRs

[Mateusz-Dobrzynski]

We're entering a stage where we have multiple repositories to take care of. In such cases the commit prefixes are usually expanded by a repository code, e.g. [DTS-70] for debate-tools, issue #70. This allows to quickly identify the commit/branch/pull request, e.g. when browsing notifications or recent activity on GitHub.

Proposed repository codes:

• debate-tools – DTS
• articles – ARCL
• tau – TAU
• sigma – SIGMA

@jakubmanczak