[Refactor] Misc. changes

- "chrome.extension" changed to "chrome.runtime" across the project
- other misc changes
[Refactor] Make function initializeForCurrentUrl() an inner function

This is because it is not meant to be called directly by itself
[Refactor/Feature] Rename function `constantize` to `getImmutableCopy` and change it to suport deep properties
[Minor] Rename `browserActionShortcuts` to `browserShortcuts` and remove unused "commands" key from manifest.json