Adding Upgrade to Enterprise version feature (#14539)

* Adding Upgrade to Enterprise version feature

* Addressing PR review comments, and adding some minor improvements

* Add tests file

* Addressing PR comments

* fix linter checks

* Storing and exposing the upgraded from TE info

* Fix showing errors on mac

* A more appropiate status code for not-supported upgrade

* Fixing tests

* Handling permissions errors

* More server logging around upgrade failures

* Apply text changes suggested from code review

Co-authored-by: Eric Sadur <57730300+esadur@users.noreply.github.com>

* Address PR review comments

* Only allow to restart the system after an upgrade

* Verify file signature before upgrade

* Adding limit to the downloaded file

* Simplifying the upgrade binary process with backup in memory

* Fixing backup/restore mechanism for the binary file

* Improve file permissions handling

* Askin the permissions for the right place (the parent directory)

* Fixing tests

* Addressing PR review comments

* Fix license headers

* Fixing retry layer

* Making it work on windows builds

* Adding license header

* Fixing 2 tests

* Fixing tests that need UpgradeFromTE System key mock

* Extracting i18n translation

* Apply suggestions from code review

Co-authored-by: Eric Sadur <57730300+esadur@users.noreply.github.com>

* Improving how the errors are written

* Fixing another error text

* Removing unneeded translation

* Fixing upgrade status strings

* Update i18n/en.json

Co-authored-by: Eric Sadur <57730300+esadur@users.noreply.github.com>

* Fixing tests

Co-authored-by: Eric Sadur <57730300+esadur@users.noreply.github.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

testlib/store.go

@@ -24,6 +24,7 @@ func (s *TestStore) Close() {
 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", "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)