add notification toggles and checks

[eibex]

Describe the PR changes

• Adds a new table called guild_settings: I would like to use this for anything guild related in the future, I tried to merge systemchannels into this but I couldn't find an elegant way yet. I will figure it out before merging... Most likely going to copy the contents into a new column and then delete the old table in the one_to_two function
• Moved the admin migration introduced by @Edwinexd from core.database to core.schema (I believe everything should still be working correctly)
• Adds a toggle command rl!notify
• Guilds are inserted into the table when either rl!notify is first used or when the bot checks for it (a user adds or removes a reaction from themselves). In the first case, the bot toggles the notification on, in the latter, the bot sets notifications to off (the "default")

I still need to test this properly, but comments are welcome

Closes #50

[eibex]

Thanks, I noticed them yesterday too but I forgot about them pretty quickly.
I pushed some extra changes to merge systemchannels into guild_settings... Modified the function to delete system channels from the database to convert the db value into a 0. I think I need to make some extra changes as

bool([(0,)]) == True

and in the if statements we only check if the list is empty or not.

EDIT: I guess it was faster to actually make the change than explaining it

[eibex]

What do you think of merging the two tables? Does it make sense not to create a new table for every single new feature?
EDIT: I'm starting to think it's not such a great idea... Managing updates, insertions and replacements is becoming a nightmare.

[Edwinexd]

I personally prefer having everything guild-specific in its own table (eg in this case system channels & notification setting ) while a bit more annoying to mange updates it is easier to remember how to get something from the database eg: I want the guild system channel I'll go read it from the guild settings table as it makes sense to be there.

While not a problem with having 1 table for each setting it requires storing the guild id multiple times in multiple tables using up unnecessary space. I know it won't use up that much space but it feels like it is better to do it now then later if it is something that wants implementation later on.

[eibex]

Alright, I will tweak the old db functions to maybe use INSERT OR IGNORE followed by an UPDATE to avoid headaches. (Since REPLACE would make values not specified NULL).

[eibex]

After testing and inspecting the DB manually this looks good to go for me. Let me know if you have any comments before merging.

[Edwinexd]

Looks good👍