Make implicit thumbnails lower priority than explicit

The previous resolution order was to use a thumbnail specified in a
notebook, then the one in the conf.py, if present. If a default
implicit thumbnail is used, it should be lower priority than these two
options.

doc/configuration.ipynb

@@ -387,7 +387,8 @@
     "(i.e. source file without suffix but with subdirectories)\n",
     "-- optionally containing wildcards --\n",
     "to a thumbnail path to be used in a\n",
-    "[thumbnail gallery](subdir/gallery.ipynb).\n",
+    "[thumbnail gallery](subdir/gallery.ipynb). Thumbnails specified\n",
+    "in notebooks will override those provided in this dictionary.\n",
     "\n",
     "See [Specifying Thumbnails](gallery/thumbnail-from-conf-py.ipynb)."
    ]

src/nbsphinx/__init__.py

@@ -496,6 +496,7 @@ def warning(msg, *args):
 
             if thumbnail_cell:
                 thumbnail['filename'] = thumbnail_filename
+                thumbnail['implicit'] = False
                 if tooltip:
                     thumbnail['tooltip'] = tooltip
 
@@ -505,6 +506,7 @@ def warning(msg, *args):
             # default to the last figure in the notebook, if it's a valid thumbnail
             if thumbnail_filename:
                 thumbnail['filename'] = thumbnail_filename
+                thumbnail['implicit'] = True
 
         resources['nbsphinx_thumbnail'] = thumbnail
         return rststr, resources
@@ -1735,15 +1737,24 @@ def has_wildcard(pattern):
                 thumbnail = app.env.nbsphinx_thumbnails.get(doc, {})
                 tooltip = thumbnail.get('tooltip', '')
                 filename = thumbnail.get('filename', '')
+                was_implicit_thumbnail = thumbnail.get('implicit', True)
+
+                # thumbnail priority: broken, explicit in notebook, from conf.py
+                #                     implicit in notebook, default
                 if filename is _BROKEN_THUMBNAIL:
                     filename = os.path.join(
                         base, '_static', 'nbsphinx-broken-thumbnail.svg')
-                elif filename:
+                elif filename and not was_implicit_thumbnail:
+                    # thumbnail from tagged cell or metadata
                     filename = os.path.join(
                         base, app.builder.imagedir, filename)
                 elif conf_py_thumbnail:
                     # NB: Settings from conf.py can be overwritten in notebook
                     filename = os.path.join(base, conf_py_thumbnail)
+                elif filename:
+                    # implicit thumbnail from an image in the notebook
+                    filename = os.path.join(
+                        base, app.builder.imagedir, filename)
                 else:
                     filename = os.path.join(
                         base, '_static', 'nbsphinx-no-thumbnail.svg')