Refactor of User model and all classes associated with it

[tsanevp]

Changes made include the following:

build.gradle:

• decreased line coverage until I figure out how to test private methods :/
• added dependency needed for @VisibleForTesting annotation

Bot.java:

• renaming the packages caused an import to change

command package:

• renamed command package to discord, now discord package has subpackages handlers, commands, events. This help people understand the structure of our codebase

GuildController.java:

• Added @Nonnull annotations and Objects.requireNonNull in various places
• added new method "verifyUserNoLongerExistsInAnyGuild()" to check if a user is a member of any guild in our guild collection. If they are not, they will be removed from the User Collection.

UserController.java:

• removed all methods related to set/get of guildId and tradingChannelId
• renamed "removeUserByMemberAndGuildId()" to "removeUserByMemberId()" since guildId is no longer associated with a user

CommandModule.java:

• this is just from the renaming and moving of classes/packages

MessageBuilderHelper.java:

• renamed from "MessageBuilder.java" to "MessageBuilderHelper.java" to give better understanding of class purpose
• Added spacing in methods to make it more readable

SettingLocationHelper.java:

• renamed from "Location.java" to "SettingLocationHelper.java" to give better understanding of class purpose
• factored "cityAndStateSetEmbed()" out of "onStringSelectInteraction" since it is testable code. It also makes "onsStringSelectInteraction" more readable and easier to understand
• Factored our "createCityMessageBuilder" to its own class for readability and testability.

CreateListingCommand.java:

• Lots of reactors here
• Mainly tried to pull out code that was testable into its own methods
• This would increase our coverage and make it more readable
• I am not sure if I went over the top though, would like some opinions and feedback.
• Still figuring out @VisibleForTesting and how to test private methods

MyListingCommand.java:

• Added private method declarations to methods since they are only used in that class
• Pull an embed out to initialize it on its own to increase readability and decrease indentation levels

UpdateLocationCommand.java:

• Pull "createUpdateLocationMessage" into its own method to test it

RemoveMemberEvent.java:

• Added check to see if user exists in another guild that has the bot in it, if yes they ARE NOT removed from the user collection. If they do not exist anywhere, they are removed.

Guild.java & Listing.java:

• Added javadoc

User.java:

• Added javadoc
• Removed guildId and tradingChannelId

[tsanevp]

Many of the changes are just the renaming of classes to make more sense.

Some of the changes I wasn't sure about are in the command classes. I've been trying to extract testable code into its own methods, but idk if I was doing toooo much.