Improve links behavior

[Fs00]

This PR improves the behavior of links in comments, releases etc. in several ways:

• directly start the corresponding OctoDroid activity when possible, instead of asking the system to open the URL (which is the URLSpan behavior).
  This avoids the need to set OctoDroid as the default handler for GitHub links, in order to prevent the Android "Open with..." sheet from showing up every time.
• when the clicked link is a GitHub link that can't be handled by OctoDroid, open it in a custom tab. In my opinion this helps reducing the perceived "switch of context" when viewing comment images.
  An alternative solution could be to open all links in custom tabs by default and add an option to toggle this behavior. I'm not sure on which of the two solutions is best.
• open custom tabs in the same task as the application. I think this is one of the most useful benefits given by custom tabs (in this context, at least).

I've also added a missing <queries> intent filter needed to be able to detect which browser support custom tabs on Android 11+.

Implementation note: the refactoring in d2dd3e8 was done in order to be able to execute the loadTask in LinkSpan, without closing the activity in which the link was clicked.