Check if update has been done during last minute

Cancel check action if update has been done during last hour.
Log error if update.freshrss.org is unreachable
Import all .json files

Before, only feed_*.json and *starred*.json was imported. Now, all
*.json files are imported.
FIXME (import/export) Use entryDAO addEntryPrepare
Import/Export: use transactions

List of articles must be iterated twice since feeds must be in DB before
using transaction for articles. It may be improved?

See https://github.com/marienfressinaud/FreshRSS/issues/591
Merge branch 'dev' of github.com:marienfressinaud/FreshRSS into dev