Fix keyboard on small mobile screens hiding composer

[SychO9]

Fixes #2628

Reviewers should focus on:
You can test this by going to nightly and adding/removing this custom CSS

@media (max-width: 767px) {
  .Composer:not(.minimized) {
    height: 550px !important; /* This makes the composer long enough to emulate your mobile keyboard hiding a part of it (in case your screen is large enough for that not to happen by default) */
    max-height: 100%; /* The actual fix, try with and without */
  }
}

Confirmed

• Frontend changes: tested on a local Flarum installation.

[dsevillamartin]

It does look like this fixes the issue 👍

[askvortsov1]

@SychO9 I'm guessing #2628 (comment) would be a separate PR (different bug), so this is probably good to merge?

[SychO9]

Yes, separate bug, we can fix that in a separate pull request.