Adds a page to the admin panel which allows an admin to upload a custom emoji 

by uploading a name, shortcode, description, and image. These fields are 

editable whenever the user re-vists the page. The emoji data is loaded from, 

and saved into SiteConfiguration, while the image data is loaded from and 

saved to DefaultStorage.

Further work that needs to be done:

* Adding labels to the form fields, and removing the extra "name" field at 

  the very top of the form. 
Summary document in Notion: https://www.notion.so/reviewboard/Custom-Emojis-Progress-6644ec34f5a84c6ab957ea11ae2b7259

Unit testing: Unit test suites for class Emoji_Widget

Local testing: making sure the app runs property with print() statements.