Support multiple timestamps in featuresearch/featureidentify

As discussed in the https://github.com/geoadmin/mf-geoadmin3/pull/1784, the feature tree and the search do not yet support multiple timestamps. This PR addresses it.

Note that it needs https://github.com/geoadmin/mf-chsdi3/pull/1083 to work. A back-end branch is setup and ready to be used for testing `gjn_multiyear_search`. Note that the chsdi3 branch needs to be merged before the timestamp branch gets into master.