fix: 3 digit hex color value in color input not supported

[imorland]

Fixes #0000

Changes proposed in this pull request:
When a 3 digit dex color is provided in a color input, the preview remains black, due to the color picker spec requiring 6 digits. To combat this, if a short hex color is provided, convert it.

Reviewers should focus on:

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)