feat: missing guild attributes

[Droid00000]

Summary

Finally got around to doing this, but this adds all of the missing server attributes and fixes a few caching bugs related to server data. Previously, Server#update_data didn't account for nullable fields, because the code incorrectly assumed that all of the fields were optional, which they aren't. Following what we did with roles, I re-factored Server#update_data. Closes #419

Added

Server#locale
Server#description
Server#application_id
Server#max_member_count
Server#max_presence_count

Server#vanity_invite_code
Server#discovery_splash_id
Server#boost_progress_bar?
Server#vanity_invite_url
Server#discovery_splash_url

Server#system_channel_flags
Server#max_video_channel_members
Server#max_stage_video_channel_members

Server#mfa_level
Server#nsfw_level

Server#rules_channel
Server#safety_alerts_channel
Server#public_updates_channel

Server#modify=
API::Server#update!

Fixed

We could end up with a stale server cache because a field that was explicitly set to nil, e.g. someone disables the system channel, would never end up being updated in the local cache.

Previously, Server#splash didn't accept an image file. I updated the code so that it can process image files as well now.