Create preferences for self-hosted goread installations (and add long-press context menu to share links)

I am hosting my own installation of Goread, but the android app was using a hardcoded value for the domain. I added a preference activity to change the URL. The default value is still https://goread.io.

If you don't want this feature in your app at all, please tell me and I'll keep it in my own fork of the repo.

One of the things I still need to change is the login attempt right after    the first start of the app - this is still using the default value    for the domain *https://goread.io*. I'm not quite sure yet how to    change that and maybe show the domain preference first... I'd be happy if you have some hints how to solve this. :smiley: 

Unfortunately I included another change in this change set: When long-pressing inside the article, a context menu pops up now to share the URL to the story or a link inside the article, if long-pressed on a URL instead of somewhere on the story text.

:question: If you want this change split out I can try to move this changeset into a new branch and create a new pull request for it.