feat: messages extension

[SychO9]

Part of the roadmap

Changes proposed in this pull request:

• Adds a new basic messaging extension.
• The models were named Dialog and DialogMessage to prevent conflicts with existing ecosystem messaging extensions that have already claimed the Conversation and Message names.
• Most of the JS side components (Dropdown, Page) are lazy loaded.
• A number of minor JS changes had to be made to other extensions (like tags) to allow reusing certain behavior.
• The notifications in core had to be changed to allow having email-only notifications, as the notification for a message received does not need to be duplicated to alerts as well.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.