Adjust boolean check of is_email_confirmed to suppress extraneous user activation events

[grimmdude]

Fixes #2713

Changes proposed in this pull request:

Instead of explicilty checking for a true value, adjust to check if is_email_confirmed evaluates to true. This matches the approach currently being used elsewhere in the User model, and will prevent the Activated event from being fired every time a user confirms their email.

Existing usage of this approach:
https://github.com/flarum/core/blob/5f110f73e7858741907665b1ec8b5c372be99796/src/User/User.php#L725-L727

Reviewers should focus on:

Activating an already activated user.

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.

[askvortsov1]

Thank you for (yet another) PR! I think this might be the most PRs we've seen from a new contributor in one day 😁. Approving pending style fixes.

[SychO9]

Thank you for the PR!

Eventually, we should use an eloquent attribute cast to boolean, but probably for a major release.