[safari] things to fix

[kouks]

• [x] safari.self x safari.application
• [x] content script being reinjected over and over again
• [x] safari storage returns null, not undefined when key is not found
• [x] event.target.tab instead of page in messaging
• [x] bind Messaging as a singleton
• [x] check if respond promise exists (both content scripts are picking up responses from the background script)
• [x] new tab is always active
• [x] add check for window.top === window before loading the script
• [x] make cli bundle the exension to {name}.safariextension
• [ ] solve relative URLs in the extension as safari appens a random string... :1st_place_medal: