Custom Facebook metatags for offers

[magul]

In #1056 we introduced default Facebook metatags, that will be set for every router change. For offer components we should introduce custom metatags (seting them inside ngOnInit).

Custom tags contains:

{ name: 'og:title', content: `${offer.title} - Volontulo. Portal dla wolontariuszy` }
{ name: 'og:description', content: offer.description }
{ name: 'og:image', content: offer.image | default image set already }

[dagaaaaa]

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