Update Encore and tweak some of its settings

[Lyrkan]

This PR:

• Updates the version of Encore to ^0.26
• Updates the related bundle to ^1.4
• Fixes the imported font paths that were not working anymore due to the recent resolve-url-loader update
• Enables integrity hashes by default
• Adds core-js@3 and configures Babel properly so polyfills are automatically added to the resulting build
• Disables file versioning when not in production mode (which prevented from using the dev-server)

[Lyrkan]

Failing tests should be fixed by #955.

[javiereguiluz]

Thanks a lot for working on all this! I tried to merge this and solve the conflicts myself, but I couldn't. Could you please solve the conflicts? Thanks.

[Lyrkan]

It should be ok now :)

[stof]

should be enabled only in production. In dev, it would cause issue when using the watcher, as I'm not sure the updated entrypoints.json gets dumped again by the watcher (and then, that would make it an integrity violation)

[Lyrkan]

It actually works fine in both --watch and dev-server mode.

For the first one, the entrypoints.json does get dumped again with updated hashes, and since --watch implies a browser refresh the integrity attribute gets updated accordingly.

Here is an example with some additional logs after changing the app.js file:

And with the dev-server enabled the integrity hashes are not computed at all since the files aren't dumped. This results in an empty integrity object in the entrypoints.json and no integrity attributes in the generated tags:

{
  "entrypoints": {
    // (...)
    "search": {
      "js": [
        "http://localhost:8080/build/runtime.js",
        "http://localhost:8080/build/vendors~admin~app~login~search.js",
        "http://localhost:8080/build/vendors~admin~app~search.js",
        "http://localhost:8080/build/vendors~app~search.js",
        "http://localhost:8080/build/vendors~search.js",
        "http://localhost:8080/build/search.js"
      ]
    }
  },
  "integrity": {}
}

[javiereguiluz]

Thank you Vincent.