Unify datetime manipulation for the multiple database engines

[fmartingr]

Data

• Shiori version: 1.6.3+ and potentially all older versions
• Database Engine: All
• Operating system: Not aplicable
• CLI/Web interface/Web Extension: Not aplicable

Describe the bug / actual behavior

Having multiple database engines means storing data in potentially different data types. That's the case with the datetime datatype. We store it differently for each database engine, returning different results in the API.

We need to add a layer of abstraction on our database or DTO structs so that the API returns the same datetime format.

We need to take into consideration that every datetime in database is UTC, since SQLite for example uses a string to store the datetimes (it does not have any time specific datatype).