[openstreetmap/openstreetmap-website] Switch CI to github actions (#3002)

[Andy Allan]

This is the first time I've dealt with github actions, so I'm not sure I'm in the best place to review! But from what I see it looks good to me, and I'm happy to use it to replace travis. Some notes:

* I was surprised to see the coveralls gem removed, but I guess whatever functionality is required is included in their action
* I don't understand the restore_key stuff for the caching, and I can't find any explanation for how restore_keys with dangling hyphens matches (prefix matching? Ordered by time or alphabetically?). However, it shows the same thing in the documentation, so :shrug: 
* I haven't found any examples of splitting up the jobs into a build stage followed by multiple parallel linting stages, using the system generated by the build stage. It seems a waste to have every job duplicating the bundle install in parallel. Perhaps there is a trick involving a build stage to install and cache, with the expectation that the cache will then be usually available for every lint stage. But I guess this is only an edge case, since they only duplicate each other on the first run after a Gemfile.lock change, so is not a big problem anyway.
* I'm also used to being able to run CI from a custom image, rather than a fixed menu of operating systems (my experience is with Gitlab CI). So I normally set up a separate repo to make custom images containing all the `apt install` prerequisites, which tend to change infrequently. That makes all the CI runs a bit quicker. But my skim of the github actions doesn't make it obvious how to do this, and in any case it looks like the github actions version of ubuntu has a bunch of stuff already installed.

So overall, a big thumbs up from me! I'm happy to see this merged if you're happy that it's ready.

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