Migrate testing pipeline to GitHub Actions

[epassaro]

Description

Say goodbye to Azure (for unit testing).

Fix and rearrange badges in README.rst - see here

Motivation and context

Closes #1953

How has this been tested?

• Testing pipeline.
• Other.

Examples

Type of change

• Bug fix.
• New feature.
• Breaking change.
• None of the above.

Checklist

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
  • (optional) I have built the documentation on my fork following the instructions.
• I have assigned and requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1985 (4b9f7a1) into master (01a018d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1985   +/-   ##
=======================================
  Coverage   59.85%   59.85%           
=======================================
  Files          70       70           
  Lines        8080     8080           
=======================================
  Hits         4836     4836           
  Misses       3244     3244           

Impacted Files	Coverage Δ
...tardis/montecarlo/montecarlo_numba/numba_config.py
tardis/tardis/montecarlo/spectrum.py
.../montecarlo/montecarlo_numba/single_packet_loop.py
tardis/tardis/io/logger/montecarlo_tracking.py
tardis/tardis/grid/base.py
tardis/tardis/analysis.py
tardis/tardis/io/config_internal.py
...s/tardis/visualization/widgets/custom_abundance.py
tardis/tardis/plasma/exceptions.py
...dis/montecarlo/montecarlo_numba/numba_interface.py
... and 130 more

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