Generate and publish code coverage reports in the CI

[Dentosal]

Closes #745

This PR sets up code coverage reporting. It commits the report to a branch called codecov-pages, which contains a folder for each branch. Branch names are urlsafe-base64-encoded to avoid (security) bugs. Every time a PR workflow runs or the master receives new commits, a CI action pushes updated codecov report there.

The testing for this feature is done in https://github.com/Dentosal/test-repo, and ported here.

Possible improvements:

• [ ] Clean up deleted branches automatically
• [x] Generate index.html that contains link to all branches
• [ ] Replace my personal name and email from the commit message with something like org-owned bot

Repository setup

The configuration has to be done by someone who has permission to edit the repo settings: set deployment from codecov-pages branch, like this:

[xgreenx]

Thank you, it works!=)