Add the API for search files (#15605)

* Adding search files api

* Fixing golangci-lint

* Adding bulk-indexing and improving a bit the name indexing for bleve and elasticsearch

* Add content extraction config migration

* Fixing a problem with document extraction

* Unapplying certain changes moved to other PR

* Fixing tests

* Making extract content app migration a private method

* Addressing PR review comments

* Addressing PR review comments

* Adding feature flag

* Removing debug string

* Fixing imports

* Fixing linting errors

* Do not migrate the config if the feature flag is not enabled

* Fix tests

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

testlib/store.go

@@ -25,6 +25,7 @@ func GetMockStoreForSetupFunctions() *mocks.Store {
 	mockStore := mocks.Store{}
 	systemStore := mocks.SystemStore{}
 	systemStore.On("GetByName", "UpgradedFromTE").Return(nil, model.NewAppError("FakeError", "app.system.get_by_name.app_error", nil, "", http.StatusInternalServerError))
+	systemStore.On("GetByName", "ContentExtractionConfigMigrationComplete").Return(&model.System{Name: "ContentExtractionConfigMigrationComplete", Value: "true"}, nil)
 	systemStore.On("GetByName", "AsymmetricSigningKey").Return(nil, model.NewAppError("FakeError", "app.system.get_by_name.app_error", nil, "", http.StatusInternalServerError))
 	systemStore.On("GetByName", "PostActionCookieSecret").Return(nil, model.NewAppError("FakeError", "app.system.get_by_name.app_error", nil, "", http.StatusInternalServerError))
 	systemStore.On("GetByName", "InstallationDate").Return(&model.System{Name: "InstallationDate", Value: strconv.FormatInt(model.GetMillis(), 10)}, nil)