Fix some forward() actions

See https://github.com/marienfressinaud/FreshRSS/issues/494
Refactor import / export source code

See https://github.com/marienfressinaud/FreshRSS/issues/494
Merge branch 'dev' of github.com:marienfressinaud/FreshRSS into dev
Fix problem with starred files for import/export

Improve guessFileType method

See https://github.com/marienfressinaud/FreshRSS/issues/494
Fix bug add favorite entry

See https://github.com/marienfressinaud/FreshRSS/issues/494