Fix Flarum 1.3 compatibility

• Fixed compatibility with Flarum 1.3+ when "number of discussions" setting is used
• Compile assets with Webpack 5

The Flarum 1.3 issue caused new posts to be evaluated like if they were new discussions when a number of discussions was required, which would send all replies for approval even if the setting only required approval for discussions. The issue was not present if the "number of discussions" setting was configured as zero.

Flarum 1.0 compatibility

Adds compatibility with Flarum 1.0

Flarum beta 16 compatibility

Adds compatibility with Flarum beta 16.