Separate topic-related functionality

[birthdaysgift]

We want to change our project layout from "package by layer" to "package by layered feature". It's a good practice and will allow us to avoid having huge files with thousands lines of code. You can read more about different project layout approaches here and here.

As a part of this we need to move all topic-related functionality to a separate directory. From this:

src/
    templates/
    __init__.py
    config.py
    database.py
    forms.py
    models.py
    routes.py

To this:

src/
    templates/
    topics/
        templates/
            some-template.html
        __init__.py
        forms.py
        models.py
        routes.py
    __init__.py
    config.py
    database.py
    forms.py
    models.py
    routes.py

So in the scope of this task we need to move all topic-related code to the separate package inside our src directory as described above. And check that everything works fine after these changes.