Expand images on tap (#315)

* Detect click on images in WebView

* First stub of the fragment to show the image in full screen

* Scale image dimension to fit the display

* Hide toolbar from Image view

* Add back button to the Image view

* Open one image on tap

* Allow zooming on images

* Revert to using Toolbar for navigation.

* Remove vibration when opening the Image view

* Do not open links associated with images

* Send all images in the webpage to the Image fragment.

* Change image on swipe

* Store article images in cache in background.

* Use PhotoView in place of WebView to display images. Implemented a pager to swipe through images.

* Removed debugging logging.

.github/CONTRIBUTING.md

@@ -41,6 +41,12 @@ Always check if the web version of your instance is working.
 * Remember that PR review can take time.
 
 
+# Install Selfoss (if you don't have an instance)
+
+I won't provide any selfoss instance url. If you want to help, but to not have one, you'll have to install one, and use it.
+
+All the details to need are [here](https://selfoss.aditu.de/).
+
 # Build the project
 
 You can directly import this project into IntellIJ/Android Studio.