Feature - add Firefox and Opera support

[soshochang]

MM uses an abstraction layer for interacting with chrome.storage and other browser specific interfaces. We could do the same to add support for other browsers(firefox, opera etc)

[soshochang]

-firefox is currently able to run qrypto, but the UI is buggy, so we are not publishing it yet. We have deprioritized this. Opera works fine with existing code, so we are going ahead and publishing it on the opera store.