Ordering listing elements

[jum-s]

Allowing users to reorder elements in a list.
Adds a ordinal field to elements documents

most important commit: 40a8d56

• Storing reorderable position have lots of possible implementations, especially with noSql databases. But until we experience performance problems, we could stick to this proposed implementation : position integer paradigm (although alphabetical rank looks fun to explore).
• ordinal naming improves grepability as position is already used by user.position = [lat, lon]
• Starts first ordinal from 0 to avoid some off-by-one mess, but at the cost of returning falsy value (aka 0)
• Script to assign ordinals may be deleted after production db update

[jum-s]

In order to test the script to fill elements without ordinal, one may:

• delete lists and elements couch databases,
• replace assert_.number(element.ordinal) by delete element.ordinal
• skip test xit('should not add twice an element already in listing' (otherwise, it will create 2 elements with same list and uri which should be forbidden)
• run tests/api/listings/
• check that no element have an ordinal key
• run scripts/db_actions/assign_elements_ordinal_to_all_listings.ts
• check that all elements have an ordinal key