Constrain classes further in MailPreview controller (#1078) #726

Job	Run time
testsuite / testsuite-linux (8.2, sqlite, lowest, false)	24s
cs-stan / Coding Standard & Static Analysis	39s
testsuite / testsuite-linux (8.2, mysql, highest, false)	54s
testsuite / testsuite-linux (8.4, mysql, highest, false)	44s
testsuite / testsuite-linux (8.5, sqlite, highest, false)	33s
testsuite / testsuite-linux (8.2, pgsql, highest, false)	45s
testsuite / testsuite-linux (8.3, sqlite, highest, false)	31s
	4m 30s

Provide feedback