Add aria-label attribute to navigation drawer button.

[grimmdude]

Changes proposed in this pull request:
Adds aria-label attribute to navigation drawer button to improve accessibility and satisfy this warning in Chrome.

Reviewers should focus on:
Affects every page that the navigation drawer button is visible.

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.

[davwheat]

Thank you for the PR!

We'll need to use Flarum's translator for this.

Could you add a key in the core.yml file inside the locale folder (make sure it's added in the correct place alphabetically), then get the text with app.translator.trans('core.<key>') in the JS.

The best place would be under core.lib:

https://github.com/flarum/core/blob/2daee924c56a258a651697f7af2cf12f6a3d50f5/locale/core.yml#L501

[grimmdude]

Got it, thanks @davwheat

[davwheat]

Oops, ariaLabel also needs to be aria-label. Missed that when looking on my phone.