Pure relocation, no behaviour change: gather all request-scoped middlewares
and their tests into dedicated files instead of scattering them across
rest.go and ssl.go.
funcs -> app/rest/api/middleware.go:
timeout (from ssl.go); rejectAnonUser, matchSiteID, cacheControl,
apiCSPMiddleware, securityHeadersMiddleware, subscribersOnly,
validEmailAuth, rateLimiter (from rest.go)
tests -> app/rest/api/middleware_test.go:
TestTimeout (from ssl_test.go); TestRest_rejectAnonUser,
TestRest_cacheControl, TestRest_apiCSP, TestRest_securityHeaders,
TestRest_subscribersOnly, Test_validEmailAuth, TestRest_matchSiteID
(from rest_test.go)
go test -race, vet, golangci-lint and govulncheck clean; example builds.