Add clear session button to Request panel #686

Job	Run time
testsuite / testsuite-linux (8.4, mysql, highest, false)	41s
testsuite / testsuite-linux (8.5, sqlite, highest, false)	24s
testsuite / testsuite-linux (8.2, sqlite, lowest, false)	28s
testsuite / testsuite-linux (8.2, mysql, highest, false)	49s
testsuite / testsuite-linux (8.3, sqlite, highest, false)	25s
testsuite / testsuite-linux (8.2, pgsql, highest, false)	47s
cs-stan / Coding Standard & Static Analysis	39s
	4m 13s

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