Followup fix for cache & global site setttings handling

[Oaphi]

This PR follows up #1517 and #1516 ensuring that:

• global site settings clear cache for all communities, not just the current one;
• adds proper methods for SiteSettingsController's actions (access?, audit_update, clear_cache, do_create methods);
• adds global? method on the SiteSetting model;
• fixes overflow of site setting values when the <details> element is opened (it's a crude fix for now via overflow-wrap - we should make HTML-valued settings use code blocks instead);

Site setting value overflow before the fix:

And lack thereof after:

[cellio]

LGTM - tested and it works. I defer to Art for code review.