Partials (#3070)

* Remove GroupDMChannels

they sparked no joy

* Start partials for message deletion

* MessageUpdate partials

* Add partials as an opt-in client option

* Add fetch() to Message

* Message.author should never be undefined

* Fix channels being the wrong type

* Allow fetching channels

* Refactor and add reaction add partials

* Reaction remove partials

* Check for emoji first

* fix message fetching

janky

* User partials in audit logs

* refactor overwrite code

* guild member partials

* partials as a whitelist

* document GuildMember#fetch

* fix: check whether a structure is a partial, not whether cache is true

* typings: Updated for latest commit (#3075)

* partials: fix messageUpdate behaviour (now "old" message can be partial)

* partials: add warnings and docs

* partials: add partials to index.yml

* partials: tighten "partial" definitions

* partials: fix embed-only messages counting as partials

src/stores/GuildMemberStore.js

@@ -180,7 +180,7 @@ class GuildMemberStore extends DataStore {
 
   _fetchSingle({ user, cache }) {
     const existing = this.get(user);
-    if (existing && existing.joinedTimestamp) return Promise.resolve(existing);
+    if (existing && !existing.partial) return Promise.resolve(existing);
     return this.client.api.guilds(this.guild.id).members(user).get()
       .then(data => this.add(data, cache));
   }