Flarum v1.2 override updates & fixing conflicts with other frontends

[dsevillamartin]

Fixes #56 Fixes #47

Changes proposed in this pull request:

• Update Flarum overrides for v1.2 (#56)
• Only apply nightmode changes to 'forum' and 'admin' frontends (#47)
• Update to Webpack v5
• Update composer.json Flarum core constraint & authors

Reviewers should focus on: Do we like this solution for the frontends? It seems to work. I don't love changing the $document->css to $head but the other solutions would probably involve changing the order of that HTML and that'd be yet another override.

Screenshot

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).