Updated flask application. Added responsive UI and search input

[slammer23]

I redesigned the UI and added responsive bootstrap4 elements.

• Added Navbar. Navbar includes home and Playlist link. Search function automatically navigates to /tracks. Navbar is in the _navbar.html and is included into base.html
• Fixed issues with 404.html, app was throwing error
• Created a ReactData class in models.py, not other changes
• Added input read to /tracks and used user input to read data from reddit. I need to dig into praw-api in order to implement a better error handling. At the moment it is just a try/except. Best would be first to check the subreddit for the existence and query details afterwards.
• Remove in template styles and replaced all style with bootstrap classes + a custom.css file
• Added /static folder to add static files like css and js
• No changes to the original code in fresh.py and models.py

[amcquade]

@slammer23 damn nice work! thanks for contributing!