GitHub actions email on failure

[manicki]

Further work to move over from Travis CI.
Includes the commit from #138

Relevant commit for emails on failure is dc3b2b9 and following ones

[manicki]

meh, think I shall try defining the email content in markdown instead of writing HTML in yaml file

[manicki]

substituting github actions context variables in a file (does not matter if markdown or HTML) does not currently work, so gotta stick to writing HTML, or maybe markdown is slightly better, in yaml file for now.

[manicki]

Figure out how to use Github action context variable in external files defining the content of CI failure emails. This PR branch will need a rebase after the "parent" PR is potentially merged any way, so keeping the markdown as a separate commit until then. It makes the env variable substitution a bit more visible and I would actually like to hear about this the most.