15-minutes survey: Provide a configurable trip segments section

[tahini]

• [x] Make sure the segments/trips objects are well defined
• [x] Bring related helper functions to evolution and test
• [x] Bring the main segment widgets to evolution: sameAsReverseTrip, modePre, mode and isNotLast, as well as all necessary functions/data. Test. Update demo_survey (PR #596)
• [ ] Bring the segment section template from od_nationale to evolution. Test. Update demo_survey
• [ ] Provide a pluggable segments group configuration, that can be directly imported and configured in individual surveys.
• [ ] Add additional configurable questions per mode to fine-tune the mode list (see the TODOs in SegmentAttributes)
• [ ] Provide a pluggable trips and segments section configuration that can be directly imported and configured in individual surveys. This should include the mandatory widgets used by the trips and segments template.
• [ ] Make sure all those widgets are configurable and can be used, or not, for ANY survey. This may require to add extensible parameters to them, like options, for the different modes, lists, choices per geographical zone, etc. Test
• [ ] Document all possible extension points.
• [ ] Make sure the audits are updated to match this new generic structure

[tahini]

Note for the reviews of the PRs involved: there should be no remaining labels using the fr/en syntax, all should be t functions. All localizable strings should be in english, no more french in Evolution. Make sure all localization strings can be overridden in surveys with an array passed to the t function, with one string always defined and the other overridable with a survey:<rest of the string>