Add SVS version control system

[tfiedor]

This PR adds SVS version control, that is made as a default:

1. SVS has single branch and single version, that tracks nothing.
2. SVS is made default and is intented for gitless usage and version-less usage of perun.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.91%. Comparing base (1d29db0) to head (df4e3f6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #216 +/- ## ======================================= Coverage 98.91% 98.91% ======================================= Files 144 145 +1 Lines 9932 9979 +47 ======================================= + Hits 9824 9871 +47 Misses 108 108 ``` | [Flag](https://app.codecov.io/gh/Perfexionists/perun/pull/216/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Perfexionists) | Coverage Δ | | |---|---|---| | [coverage-3.10](https://app.codecov.io/gh/Perfexionists/perun/pull/216/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Perfexionists) | `98.90% <100.00%> (-0.01%)` | :arrow_down: | | [coverage-3.11](https://app.codecov.io/gh/Perfexionists/perun/pull/216/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Perfexionists) | `98.91% <100.00%> (+0.01%)` | :arrow_up: | | [coverage-3.12](https://app.codecov.io/gh/Perfexionists/perun/pull/216/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Perfexionists) | `98.90% <100.00%> (+<0.01%)` | :arrow_up: | | [coverage-3.9](https://app.codecov.io/gh/Perfexionists/perun/pull/216/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Perfexionists) | `98.90% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Perfexionists#carryforward-flags-in-the-pull-request-comment) to find out more.

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