global: theme: add config var to define site name #1222

Job	Run time
Tests (ill, 3.9, pypi, postgresql14, opensearch2)	4m 11s
Tests (ils, 3.9, pypi, postgresql14, opensearch2)	13m 26s
Tests (acquisition, 3.9, pypi, postgresql14, opensearch2)	2m 26s
Tests (circulation, 3.9, pypi, postgresql14, opensearch2)	8m 50s
Tests (document_requests, 3.9, pypi, postgresql14, opensearch2)	3m 19s
	32m 12s

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