Upgrade to webpacker 6

[ferblape]

We are having lots of issues with the Terser webpack plugin in CircleCI, which is causing we got a lot of failures that we tend to ignore which produce that we push to master failing tests.

Webpacker 6 is almost ready, with webpack 5 which has a default terser and doesn't require the plugin anymore

Upgrading notes: https://github.com/rails/webpacker/blob/master/docs/v6_upgrade.md

Also, remove the terser plugin and let's see if your trouble with CircleCI gets fixed

[Crashillo]

In order to include PurgeCSS some tips we ought to keep in mind:

• Previous PurgeCSS versions were purging all dynamic styles. That is, if your code has a dynamic import: import("../path/to/lib") and such lib file has any style on it, purgecss will rip that out, because is not present at compilation time.
• Libraries creating dynamic HTML, like d3 or leaflet, will be purged also. That's due to, at compilation time, there's no yet the markup which the styles refer to, hence it's removed.

Currently there's a safelist (both config or comments in code) to avoid this removal.

There's a chance this issues will be fixed in webpack v5 (webpacker v6)