global: theme: add config var to define site name #1221

Job	Run time
Tests (circulation, 3.9, pypi, postgresql14, opensearch2)	8m 13s
Tests (document_requests, 3.9, pypi, postgresql14, opensearch2)	3m 43s
Tests (ill, 3.9, pypi, postgresql14, opensearch2)	4m 0s
Tests (acquisition, 3.9, pypi, postgresql14, opensearch2)	2m 27s
Tests (ils, 3.9, pypi, postgresql14, opensearch2)	12m 48s
	31m 11s

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