init: no error if examples/ dir cannot be found; overhaul repo_icon

There are so many possible installation paths for Python modules, it has
been very hard to even find and test them all.  This adds a fallback option
if the examples dir cannot be found.  A repo can work without an icon or
the example config.py.

This removes the fake assumption that the icon can be a full path in the
config.py.  While the path was being properly passed through to the index
file, the file was never copied properly into place nor rsynced to the web
server.

examples/config.yml

@@ -54,7 +54,6 @@
 
 repo_url: https://MyFirstFDroidRepo.org/fdroid/repo
 repo_name: My First F-Droid Repo Demo
-repo_icon: fdroid-icon.png
 repo_description: >-
   This is a repository of apps to be used with F-Droid. Applications in this
   repository are either official binaries built by the original application
@@ -68,10 +67,18 @@ repo_description: >-
 archive_older: 3
 archive_url: https://f-droid.org/archive
 archive_name: My First F-Droid Archive Demo
-archive_icon: fdroid-icon.png
 archive_description: >-
   The repository of older versions of packages from the main demo repository.
 
+# The repo's icon defaults to a file called 'icon.png' in the 'icons'
+# folder for each section, e.g. repo/icons/icon.png and
+# archive/icons/icon.png.  To use a different filename for the icons,
+# set the filename here.  You must still copy it into place in
+# repo/icons/ and/or archive/icons/.
+#
+# repo_icon: myicon.png
+# archive_icon: myicon.png
+
 # This allows a specific kind of insecure APK to be included in the
 # 'repo' section.  Since April 2017, APK signatures that use MD5 are
 # no longer considered valid, jarsigner and apksigner will return an