Read context improvements

This is a collection of small issues connected to the Read folder and views within the context of reading emails:

• Make floating buttons in MessageMainView circles. Currently they're not always circular.
• Remove redundant "Attachments:" title from AttachmentsMainView
• Fix naming conventions in Read folder. Is "Main" always necessary? Views should end in View, i.e. ❌MessageViewMain, ✅Message(Main?)View