Hide empty entries

[pmartin]

Sometimes, wallabag doesn't know how to extract content from a entry -- which leads to something we cannot read in wallabag nor in the application.

This is frustrating for users: we have entries, sometimes a few, we cannot read in the application.
=> Hide those entries

To detect those entries, two ideas:

• Their estimated reading time seems to always be 0; but risk of false positives for short entries?
• Their content looks a bit like "wallabag cannot fetch this"