MM-21552: Adding SaveMultiple to posts (#13766)

* Adding SaveMultiple to posts

* Improving tests

* fixing i18n

* Fixing tests

* Improving testing on top of Save and SaveMultiple

* Fixing shadow variables

* Addressing some PR comments

* More clear update post test

* Addressing some PR comments

* Addressing some PR comments and simplifying the code

* Improting replies in bulk too

* Fixing reply count and processing last imported replies

* Adding OverwriteMultiple to posts aggregating everything in the same transaction

* Adding 2 pending tests to implement

* Adding tests for overwrite multiple posts

* Adding tests for TeamStore.GetByNames method

* Fixing shadow variables

* Addressing PR comments

* Extracting i18n strings

* Fixing tests

* Fixing tests

* Adding more test cases

* Using a variable instead of a fake timestamp

app/import_test.go

@@ -239,12 +239,12 @@ func GetAttachments(userId string, th *TestHelper, t *testing.T) []*model.FileIn
 
 func AssertFileIdsInPost(files []*model.FileInfo, th *TestHelper, t *testing.T) {
 	postId := files[0].PostId
-	assert.NotNil(t, postId)
+	require.NotNil(t, postId)
 
 	posts, err := th.App.Srv().Store.Post().GetPostsByIds([]string{postId})
 	require.Nil(t, err)
 
-	assert.Equal(t, len(posts), 1)
+	require.Len(t, posts, 1)
 	for _, file := range files {
 		assert.Contains(t, posts[0].FileIds, file.Id)
 	}