feat: configurable notifications

[iamwehi]

Closes #430.
Introduces new section in the config:

[notifications]
# replaces notifications = true
enabled = true

# whether to show message text
show_message_text = true

# whether to show message author/chat
show_message_chat = true

# whether to show notifications on new reactions
show_reactions = true

# whether to bell on reactions
mute_reactions_bell = false

Default values (as in the example above) provide the same behavior as notifications = true did.