Propose the database module's APIs and basic structure

[johnding1996]

• [x] Analyze the basic structure of database module
• [x] Refine it and plot a diagram
• [x] Propose the list of APIs on GitHub wiki
• [x] Review

[johnding1996]

The database APIs is on wiki page https://github.com/johnding1996/COMP3111H-Group/wiki/Database-APIs

[thomaszhouan]

user_id in Database API is int, but the LINE API is actually using String

[johnding1996]

@thomaszhouan what kind of string are you using for user_id? hashes?

[johnding1996]

Closed since out-of-dated. I'll open another issue to review the database APIs for version 1.1.