Jormungandr: fix journeys links

some list param were missing in links

small refactoring  of the links

this refactoring should be more complete, but we cannot do this before
the v2 and some thought about the internal link handling
Jormungandr: add tests on datetime_represent
Jormungandr: update create_link comment
Jormungandr: change description to title in links
Merge pull request #667 from antoine-de/prev_next_fix

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