Separate index-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 index page 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/
    index/
        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 index page related code to the separate package inside our src directory as described above. And check that everything works fine after these changes.