Fixes for link handling regressions

[Fs00]

This PR fixes a couple of regressions introduced by #1118:

• many non-GitHub links erroneously resolved to OctoDroid activities, because the LinkParser didn't take into account the possibility of receiving non-GitHub URLs
• tapping on links to relative URLs (such as anchor links in Readme) made the app crash due to an NPE
• when an URL couldn't be resolved to an OctoDroid activity, it wasn't opened in the browser as it should have been

Also, tests didn't compile until I fixed them. Ideally there should be a pipeline that builds the app and runs them, so that at least they have a chance to run from time to time :)

[Fs00]

Update: I've found another regression, I'll look into fixing it tomorrow.

[maniac103]

Let me guess: links to external URLs being broken, like your video in #1131? If not, I'd like to mention that one as well ;-) I unfortunately haven't had time to debug that one yet.

[Fs00]

Let me guess: links to external URLs being broken

Nope! That one has been fixed in 07ba31b, hasn't it?
It's a more hidden and subtle issue, you'll see it tomorrow 😉

[Fs00]

Pushed the fix for the extra regression (I've added it to the PR description).
You could experience the regression when tapping on "Roadmap" and "Releases" links in this README.