Refactor code into modules (WiFi, Slack, deep sleep, NTP, etc.)

[willblev]

To make it easier to test and debug each part of the code, we can break up the main.cpp file into separate modules; the setup
and loop sections in main.cpp would call the functions from the other modules

For instance, we could divide main.cpp into the following modules:

• WiFi_management .cpp/.h (all the WiFi and web portal related code, not sure if the FS management should be a separate module in and of itself)
• button_presses .cpp/h
• Slack_status .cpp/h (query/send status&presence&DnD, most recent remote status)
• NTP_status .cpp/h (query NTP server, most recent time)
• local_status .cpp/h (LED state, most recent current status)
• deep_sleep .cpp/h (functions to sleep/wake board at night)

Perhaps we don't need 2 files for each module, but I saw there are some precedents on what things should go where.

[willblev]

I am going to try to follow this guide and start to modularize the code in the branch refactoring-into-modules. First stage will be to split it up, one component at a time, without breaking the current functionalities. Then I can start to tidy & optimize. Maybe this one is easier to follow.

[willblev]

This is harder than I thought, but I am learning a lot! I still am having some issues deciding scopes i.e. where some values should live- since some times being a private value inside a class which can be returned with a public-facing function makes more sense, but in other cases it seems easier to just initialize a variable in main.cpp and leave it open to everything... This will take some time, so maybe I can do several PRs for step-by-step progress.