* Fix swapped parameters in ArgumentException and ArgumentNullException cstrs

The constructors of ArgumentException and ArgumentNullException are inconsistent with each other, which results in their parameters being swapped here and there.

* Use named parameters for ArgumentException constructors

Cleans up some of the methods of EmbedBuilder to use simpler syntax as well

+ Add introductory paragraphs to each FAQ section.
+ Add 'missing dependency' entry to commands FAQ.
* Split commands FAQ to 'General' and 'DI' sections.

This commit rewrites the remarks section of GetMessagesAsync, as well as adding examples to several methods.

+ Geez, I didn't know there were so many.

* Change all Select(... as ...) to OfType

* Add changes according to 194a8aa427

* Add input validation for bot tokens based on their length

* Add token validation to BaseDiscordClient#LoginAsync

Adds a TokenUtils class which is used to validate that tokens are correct

* Revert changes to DiscordRestApiClient

* Add Unit tests to the TokenUtils class, fix a logic error that was caught by those tests

* Allow for API to throw exceptions

Moves the validation of tokens to be inside of LoginInternalAsync, and writes a Warning to the console when the supplied tokens are invalid

Should fix issues in audit logs when deserializing overwrites and
similar types which we use a custom contract resolver for.

Nullable<T> is not valid for Attributes, this is my suggested fix.

Added some miscellaneous missing xmldocs

Added some REST documentation

* Don't attempt instantiation of an abstract module
* Attempt associating a TypeReader in case one is registered late (ie. OnModuleBuilding)

...because it isn't a class.