Added 3-second delay between README and solution commits so that GitHub includes both in contribution graph

Fixed bug which prevented users from connecting existing repositories with an underscore in the name

Added pagination logic so that a user's repositories beyond the first 30 can be found

Bug fixes:
- Prevent duplicate commits when solution content is unchanged
- Remove unintentional leading blank lines that occur when submitting solution via CMD + Enter

Improved code organization:
- Refactored background.js: created descriptive variables and helper functions, simplified conditional statements, and moved the repo and github management logic to separate files
- Refactored popup.js and welcome.js and moved the UI related functions to a separate file
- Modified step 3 and step 4 of the "Developing Locally" section of CodeHub's README.md
- Added and modified error handling for various functions

Added functionality:
- Create a new repo and a directory simultaneously
- Connect to a directory within an exisiting repo
- Update solution file with the most recently submitted solution code
- Create a new repo or connect to an existing repo by pressing the enter key
- Minor UI updates to welcome.html and popup.html

Fixed bugs:
- PRevent duplicate API requests when creating a README.md in a directory that already has one
- Handled issue of hidden sections being displayed during UI updates