Several code quality improvements to Discord files

[cbrxyz]

This PR adds several improvements throughout the following files:

• censor.py
• devtools.py
• embed.py
• funcommands.py
• globals.py
• logger.py
• membercommands.py
• ping.py
• reporter.py

Notably, some of the improvements include:

• The encapsulation of all MongoDB-related functions into a MongoDatabase class, of which an instance is stored in the main PiBot class.
• The fixing of several errors output by pylint, focusing on long lines and docstrings
• The addition of docstrings throughout several files, classes, and methods
• SLASH_COMMAND_GUILDS is now an array by default, with the support for multiple guilds (as the name implies)

This PR also updates requirements.txt.