Enable searching courses by teacher name

[HydrogenC]

Enable searching by teacher name on server side, resolves DanXi-Dev/DanXi#375.
In order to make the new code work, there are something changes to do on the database.

• Run the following SQL code to create the teacher table (for the sake of query performance)

CREATE TABLE `teacher`  (
  `id` bigint NOT NULL AUTO_INCREMENT,
  `name` varchar(255),
  PRIMARY KEY (`id`)
)

CREATE TABLE `teacher_course_groups`  (
  `teacher_id` bigint NOT NULL,
  `course_group_id` bigint NOT NULL,
  PRIMARY KEY (`teacher_id`, `course_group_id`),
  FOREIGN KEY (`course_group_id`) REFERENCES `course_group` (`id`),
  FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`)
)

• Run this go script to insert data into table teacher and teacher_courses from course
  Link: https://github.com/HydrogenC/backend/blob/teacher-name/danke_utils/teacher_table.go

[HydrogenC]

Also included a fix for #134.

[HydrogenC]

Resolved.

[HydrogenC]

Do I have to separate a new API for teacher name searching and use db.Table("teacher").Preload("CourseGroups").Find(...) to find related course groups?

[JingYiJun]

Do I have to separate a new API for teacher name searching and use db.Table("teacher").Preload("CourseGroups").Find(...) to find related course groups?

或许这样是更好的，不过需要前端适配两种不同的搜索模式。现在这样的策略对于用户来说，搜索 Teacher 但是显示 CourseGroup 感觉比较奇怪😂

[HydrogenC]

Do I have to separate a new API for teacher name searching and use db.Table("teacher").Preload("CourseGroups").Find(...) to find related course groups?

或许这样是更好的，不过需要前端适配两种不同的搜索模式。现在这样的策略对于用户来说，搜索 Teacher 但是显示 CourseGroup 感觉比较奇怪😂

Is there an advantage for using Preload over Join? Should I modify my current implementation then?

[JingYiJun]

Do I have to separate a new API for teacher name searching and use db.Table("teacher").Preload("CourseGroups").Find(...) to find related course groups?

或许这样是更好的，不过需要前端适配两种不同的搜索模式。现在这样的策略对于用户来说，搜索 Teacher 但是显示 CourseGroup 感觉比较奇怪😂

Is there an advantage for using Preload over Join? Should I modify my current implementation then?

Maybe in this situation Join is better since Join means only one load from database while Preload implies 3 loads, and there is no need to get the intermediate information of teachers. If there you add a new api to load the teachers user search, Preload may better, but in this pr you've done enough on changing SearchCourseGroups.