Use Webpack code-splitting to lazy load map bundle

[bbecquet]

Advantages:

• simpler webpack config, map is now part of the main bundle, but still loaded asynchronously on start-up via a dynamic import (see https://webpack.js.org/guides/code-splitting/#dynamic-imports)
• simpler code, no need for a custom file loader (but it's still used to load language file so I didn't remove if from index.ejs)
• the total size downloaded is smaller because Webpack can now reuse code already imported in the parent bundle when loading the map part

Number proof

Before

                             Asset       Size            Chunks             Chunk Names
                         bundle.js    972 KiB              main  [emitted]  main
                     bundle.js.map    799 KiB              main  [emitted]  main
                masq-lib.bundle.js   1.04 KiB          masq-lib  [emitted]  masq-lib
            masq-lib.bundle.js.map  430 bytes          masq-lib  [emitted]  masq-lib
        vendors~masq-lib.bundle.js    766 KiB  vendors~masq-lib  [emitted]  vendors~masq-lib
    vendors~masq-lib.bundle.js.map    854 KiB  vendors~masq-lib  [emitted]  vendors~masq-lib

[...]

         Asset      Size  Chunks             Chunk Names
        map.js   1.4 MiB    main  [emitted]  main
    map.js.map  1.24 MiB    main  [emitted]  main

==> bundle + map = 972 + 1400 => 2372 KiB

After

                             Asset       Size            Chunks             Chunk Names
                         bundle.js    972 KiB              main  [emitted]  main
                     bundle.js.map    799 KiB              main  [emitted]  main
                     map.bundle.js    144 KiB               map  [emitted]  map
                 map.bundle.js.map   82.6 KiB               map  [emitted]  map
                masq-lib.bundle.js   1.04 KiB          masq-lib  [emitted]  masq-lib
            masq-lib.bundle.js.map  430 bytes          masq-lib  [emitted]  masq-lib
             vendors~map.bundle.js    816 KiB       vendors~map  [emitted]  vendors~map
         vendors~map.bundle.js.map    779 KiB       vendors~map  [emitted]  vendors~map
        vendors~masq-lib.bundle.js    766 KiB  vendors~masq-lib  [emitted]  vendors~masq-lib
    vendors~masq-lib.bundle.js.map    854 KiB  vendors~masq-lib  [emitted]  vendors~masq-lib

==> bundle + map + vendors~map = 972 + 144 + 816 => 1932 KiB

[amatissart]

Nice! This will also help to remove use of events in the future, and make the code simpler.

A rebase in needed to solve a conflict in the .eslintignore

[amatissart]

For the record, bundle sizes in "production" mode:

                     Asset       Size  Chunks                    Chunk Names
                 bundle.js    311 KiB       0  [emitted]  [big]  main
             map.bundle.js   33.8 KiB       1  [emitted]         map
        masq-lib.bundle.js  160 bytes       2  [emitted]         masq-lib
     vendors~map.bundle.js    808 KiB       3  [emitted]  [big]  vendors~map
vendors~masq-lib.bundle.js    336 KiB       4  [emitted]  [big]  vendors~masq-lib