+ GameAsset
+ GameTimestamps
+ Format

* Fix comments to comply with third-person commenting style
+ Various attributes
+ Various Command-related objects
+ Many more

+ ActivityType
+ IAttachment
+ ChannelPermission
+ ChannelPermissions
+ ChannelType
+ Attachment

+ EmbedBuilderExtensions
+ CacheMode
+ ImageFormat

+ DiscordConfig
+ CDN
+ MentionUtils
+ LogMessage
+ LogSeverity

+ Modified the headings to fit the formal aspect of docs.
+ Added more details regarding RunMode

This resolves #987

Previous behavior was that even if `null` was passed for an embed in
UploadFileAsync, the Embed property on UploadFileArgs was still
specified - this meant we were always sending a payload_json.

If a payload_json is specified, it seems like Discord will only read
from the payload_json, and will ignore properties set outside of it.

To prevent unnecessary code duplication, this commit always specifies
parameters in the payload_json, and also will only include the embed if
one was actually specified with real data (not null).

- As well as making minor adjustments here and there

- The link never worked in the first place and is rarely used.

* UnixTimestampConverter should now obey a maximum value

This change prevents an issue where the converter would be unable to
handle obscenely large timestamp values - which are actually quite
common on Discord.

OptionalConverter had to be rewritten to support checking whether or not
an InnerConverter returned an Optional. The perf impacts from this
_shouldn't_ be too bad, as types without a custom parser (which should
be the majority of Optionals in the lib) will bypass the type-check.

* optimizations on OptionalConverter

- Add a helper under CDN for cover art URLs
It would be bad practice of us to leave CDN urls hardcoded in the
deserializer, would be harder to change down the line should Spotify
ever change their CDN.

I'm not entirely supportive of leaving Spotify's CDN hardcoded in our lib
either, but there's no better alternative.

- Change SpotifyGame#Artists to an IEnumerable<string>
Seems pretty common to prefer IEnumerables in place of Arrays.

* Initial Spotify support

* Remove GameAsset#ToEntity

- appId doesn't seem to be necessary, and Spotify Game doesn't return appId either.

* Implement SpotifyGame details

* Implement song Duration prop

* Add album art CDN

* Fix ActivityType

* Remove payload debug

* Add changes according to review

+ Make `ApplicationId` nullable
+ Move ctor after props

Previously it was implemented under Discord.Net.Rest, which seems
inconsistent and unnecessary.

This also allows Commands docstrings to reference EmbedBuilder, since
Commands only has a dependency on Core.

To reply with just a rich embed, users have to invoke ReplyAsync with
`ReplyAsync("", embed: embed)`, which seems wasteful, when they only
need to specify the embed.