[#79285010] Change the 'TestsController' for 'TopicsController'
[#79374398] Add relationship between Topics
[#79285010] Add 'has_content' to Topics
[#79377110] Create model 'Video' and 'Exercise'
[#79386866] Create the model 'Keyword'
[[#79386866] Create the association between Topic and Keyword
[#79388478] Create association between Topic and Video/Exercise
[#79390910] Remove 'kind' and add 'internal_id' to Topic / Change internal_id's type for string
[#[#79374398]] Fix association between Topics
[#79394216] Rename 'has_content' for 'is_node' / Set default value to 'false'
[#79285010] Get topics form a root topic (first version)
[#79285010] Implement helper's functions to get topics form a root topic
[#79285010] Resolve conflicts