Add Content for User page, preload user & throw 404 accordingly

[dsevillamartin]

Fixes #1846

Changes proposed in this pull request:

• Preload user
  • Tested it works correctly, API call to /api/users/ no longer needed as preloadedApiDocument() call adds it to the forum payload
• Return 404 if user is not found

Reviewers should focus on:
Doesn't add a no-JS view. I assume we want one, not sure about what items to include though. Posts/discussions would be hard, and require backend handling of those routes as well.

Screenshot

Confirmed

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

[dsevillamartin]

Applied some changes and added a check for user ID instead of just the username when preloading.

Do we want to throw ModelNotFoundException instead of RouteNotFoundException? I feel like the current implementation could be confusing, as the users route exists but the user model specified doesn't. And the API endpoint throws the ModelNotFoundException, so we should probably be consistent there.

[franzliedke]

Agreed. The model is not found, so we should use that exception.

[dsevillamartin]

@franzliedke Switched exception thrown 👍

[franzliedke]

Beautiful, thank you!