Issue #3605432 by hydra, TomekZiaja: Add title site setting and generate id from it

Pascal Crottrequested to merge
issue/sites-3605432:3605432-add-title-setting into 1.x

Adds a 'title' setting to the 'sites' SiteSetting plugin, a required Title field on the site settings form (defaulting to the entity label), and uses it as the machine-name source for the site id on new sites. The label sync itself lives in sites_group (#3591238); FormAlterHooks from the original MR is intentionally omitted.

Adds a unit test asserting the title setting is defined.

Closes #3605432

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