Editor: Imgur Image Upload

[zsh-eng]

Add an additional Upload callback that uses imgur instead.
This requires #11 to be completed first.
Use imgur's API for handling image uploads because it allows for up to 1250 uploads per day.

https://apidocs.imgur.com/#rate-limits

Here's the rough flow for image uploads.

1. User authenticates with imgur
2. Callback is sent to backend and imgur access token is stored in the database
3. Image is uploaded and sent to the backend, forwarded to imgur API and the image url is returned to the frontend to be displayed to the user.

Useful Resources:

https://github.com/addozhang/obsidian-image-upload-toolkit
https://github.com/renmu123/obsidian-image-auto-upload-plugin
https://github.com/gavvvr/obsidian-imgur-plugin