lint & a11y fixes

[what-the-diff[bot]]

• Added a button style to the App.css file
• Changed the text in EmptyPanel component from "Click a friend..." to "Loading up tweets"
• Updated Sidebar CSS styles for selected friends and unread messages, as well as added some new styling properties like font-size and border radius
• In Sidebar JSX, changed divs into buttons so that they can be clicked on with keyboard navigation (tab) instead of just mouse clicks; also updated className property values for readability/consistency
• Updated TweetPanel CSS styles by adding margin-top: 5px to .tweet-panel-tweet class selector; also removed width: 100% from .tweet-panel-tweet-content because it was causing horizontal scrollbars when there were long lines of text in tweet content (e.g., URLs); replaced box shadow with border radius since I think it looks better without shadows now that we have borders around each tweet panel item; finally, updated header padding and margins so that everything is centered properly within the header area
• In TweetPanel JSX, made changes similar to those done in #4 above - converted spans into buttons where appropriate so that users can click them using tab key instead of only being able to use their mouse pointer or touch screen; also fixed an issue where Mark Read button wasn't showing up if all tweets had already been marked read previously (i'm not sure why this worked before but i guess something must've changed somewhere else?)