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