Add short name field in General Settings when Site Title is longer than 12 characters

[westonruter]

Fixes #210.

This adds a new Short Name field to the General Settings screen:

This field is required if the Site Title is longer than 12 characters:

If if the Site Title is less than or equal to 12 characters long, then the placeholder value is set to match the Site Title (since it is used by default in the manifest in this case), and the field is not required:

If the Site Title is greater than 12 characters in length, a user must supply a Short Name to save the form (and the field has a maxlength of 12 supplied, which is enforced by the sanitize_callback):

If a site is supplying the Short Name via a plugin, like as follows (which was the only way to do so before now):

add_filter( 'web_app_manifest', static function ( $manifest ) {
	$manifest['short_name'] = 'WP Dev';
	return $manifest;
} );

Then the Short Name field is disabled in the same way the Site URL and Home URL fields if they are defined with constants:

The Site Health test for the Short Name now includes a link to the General Settings screen to manage the field:

Other changes:

• Use mb_strlen() for short_name check.
• Use trim() on a short name before checking the length or adding to the manifest.
• Enable client-side form validation on the General Settings screen. (It was disabled for a bug in an 8-year old version of Firefox, per TRAC-22183.)

[westonruter]

Still need to add tests, but otherwise this is ready for review.

• Add tests.

[codecov-commenter]

Codecov Report

Merging #691 (0e850c7) into develop (928905e) will increase coverage by 2.30%.
The diff coverage is 83.95%.

@@              Coverage Diff              @@
##             develop     #691      +/-   ##
=============================================
+ Coverage      16.75%   19.05%   +2.30%     
- Complexity       305      324      +19     
=============================================
  Files             55       55              
  Lines           2011     2062      +51     
=============================================
+ Hits             337      393      +56     
+ Misses          1674     1669       -5     

Flag	Coverage Δ
php	19.05% <83.95%> (+2.30%)	⬆️
unit	19.05% <83.95%> (+2.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wp-includes/class-wp-web-app-manifest.php	73.88% <83.95%> (+14.19%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 928905e...0e850c7. Read the comment docs.

[westonruter]

• Add link from Site Health test to Short Name field on General Settings screen.

[thelovekesh]

LGTM 👍