Add a pass-through API route to fetch the user's guilds

[matthew-robertson]

Is your feature request related to a problem? Please describe.
As a user of the web dashboard, or as a third-party developer, I'd like to be able to know which of my servers already have the bot on it, or could have the bot added to it.

Describe the solution you'd like
In order to display this information on the front end, it needs to be calculated on the API's end first. The simplest way to do so is to have it fetch the user's servers, filter to ones they could control the bot on, and map them to include an invite_link if the bot isn't already on it.