Implement "create sub task" action

[opatry]

• Properly dealing with child tasks requires to sync remote parent task id as local ones first (see Sync remote parent task id as local parent task id #141)
• Implement local side
• Implement remote side
• Update and add relevant unit tests
• Enable integration on UI side and update UI tests

[opatry]

• Remote to local parent task id mapping will be addressed in a dedicated task (Sync remote parent task id as local parent task id #141) link to the meta "sync" task fix issue (Fully implement and test sync #140).