Convert Travis workflow to GH actions

[robertopreste]

Closes #553

This PR replaces Travis CI with GitHub Actions.
At the moment, some Selenium tests are failing, will need to investigate why.

[codecov-commenter]

Codecov Report

Merging #554 (93527ce) into master (15d2aeb) will decrease coverage by 0.16%.
The diff coverage is 40.00%.

❗ Current head 93527ce differs from pull request most recent head d8ec906. Consider uploading reports for the commit d8ec906 to get more accurate results

@@            Coverage Diff             @@
##           master     #554      +/-   ##
==========================================
- Coverage   95.26%   95.10%   -0.17%     
==========================================
  Files          86       92       +6     
  Lines        4015     4168     +153     
  Branches      252      269      +17     
==========================================
+ Hits         3825     3964     +139     
- Misses        136      144       +8     
- Partials       54       60       +6     

Impacted Files	Coverage Δ
remoteappmanager/docker/tests/test_container.py	100.00% <ø> (ø)
remoteappmanager/docker/container.py	90.47% <40.00%> (-4.53%)	⬇️
remoteappmanager/tests/test_user.py	100.00% <0.00%> (ø)
remoteappmanager/tests/test_application.py	100.00% <0.00%> (ø)
remoteappmanager/user.py	100.00% <0.00%> (ø)
remoteappmanager/utils.py	81.35% <0.00%> (ø)
remoteappmanager/__init__.py	100.00% <0.00%> (ø)
remoteappmanager/application.py	100.00% <0.00%> (ø)

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[flongford]

Thanks for this @robertopreste! In general LGTM - my questions are mainly around understanding the changes to the container ports handling.