Javascript refactoring

[BirdyYao]

Key points:

1. apply idea of separation of concerns - split fetch posts from display posts.
2. apply idea of factory pattern - add a basic factory for initializing fetcher and display in terms of different instances.

Pros:

1. increased modularity and cohesion
2. should be able to "plug-in" new fetchers and displays for other instances
3. easier for writing unit test since separation of concerns

Cons:

1. existing unit test need to be refactored
2. lakshya's improvement on slow loading of Mastodon posts won't work since total separation of fetch and display.

TO-DOs:

1. unit testing
2. fix slow loading of Mastodon posts

[sapiosaturn]

noticed the folder is called fecther, small issue but should fix that