Revert "Issue #1974210 by disasm, larowlan: Convert admin/structure/forum to a Controller."

This reverts commit e18109e284f2e6b931e7d6d06b40e88337f18296.

Seems to be causing testbot failures. Rolling back for now.
Issue #2085989 by longwave: Correct mindless copy/paste error in SystemController docs
Issue #2083943 by Xano, InternetDevels, longwave: Fix spelling errors in various documentation
Issue #1969970 by Gábor Hojtsy: Add context information to documentation of Translation annotation
Issue #2073901 by xtfer, larowlan: document that entity_load now accepts a mixed argument for ID
Issue #2057131 by olli: Fix docs for lock wait function
Issue #2030713 by ifrik: Improve help for the email module
Issue #1974210 by disasm, larowlan, jibran: Convert admin/structure/forum to a Controller.
Issue #2084057 by dawehner: Fixed Translate _title on routes.
Issue #2087449 by andypost: Fixed broken tests FieldAttachOtherTest->testFieldAttachCache().
Issue #1963394 by tim.plunkett, jibran, effulgentsia: Fixed ConfirmFormBase::getCancelPath() should allow for a route.
Issue #2088119 by David_Rothstein: Remove David_Rothstein from MAINTAINERS.txt for Drupal 8.
Issue #2083811 by yched, plach: Remove langcode nesting for fields in entity  structures.
Issue #1987718 by brianV, larowlan: Convert form_test_two_instances() to a new style controller.
Issue #2082117 by alexpott: Fixed Install default config only when the owner and provider modules are both enabled.
Revert "Issue #2082117 by alexpott: Fixed Install default config only when the owner and provider modules are both enabled."

This reverts commit 4c304cb8e794cb6d566d4a249ee2bd5306648969.
Issue #2082117 by alexpott, xjm, tim.plunkett, heyrocker, beejeebus, sun: Fixed Install default config only when the owner and provider modules are both enabled.
Issue #2087551 by donquixote: ViewsHandlerDiscovery: Remove ->pluginNamespaces setup from constructor.
Issue #2087725 by donquixote: Block module tests in tests/lib/Drupal instead of tests/Drupal.
Issue #2084309 by amateescu: Enable Entity reference in the standard profile.