Task: General UI Updates

[alexmdodge]

In general there are some useful UI updates to additionally aid developers while reviewing pull requests:

• [x] ~~Add a simple checkbox in the file header which can be clicked to indicate review of that file is complete~~ No longer required since the viewed feature was added
• [x] Note when the viewed box has been selected and cross out a menu item accordingly
• [ ] Add accompanying state within the file explorer to display review files
• [ ] Add ability to distinguish whether a file was deleted or not to help in reviews
• [ ] Add comment icons which update the count dynamically through pull request reviews