Hide "Star Gitfy on Github" if project has already been starred

[TimMikeladze]

Hide "Star Gitfy on Github" if project has already been starred. Alternatively, to avoid Github API calls make the text visibility a toggle in the settings. Set it to visible by default.

[charlietomsett]

@ekonstantinidis please

[manosim]

Ready for next release! 🎉 🎉 🎉

Gitify checks if the user has starred the repo when the app launches and hides the message conditionally.