Major search improvements

[skalee]

Currently, every site has its own search feature implementation, which has been roughly created this way: 1) copy script from tc211 2) do some changes. This is going to be problematic. The search feature should be generalized, so that all sites can share the same script.

In particular, generalized search feature should support:

• indexing multiple designations in a single concept (e.g. multiple designations in IEV)
• term IDs other than numbers (e.g. 103-01-03 in IEV)
• filtering by some criteria (e.g. "valid only" checkbox in TC211)
• sorting by some criteria (e.g. Possibility to sort columns in search filter? isotc211.geolexica.org#130)
• colouring search results basing on their properties (e.g. https://github.com/geolexica/isotc211.geolexica.org/issues/129)

[ronaldtse]

Strongly support this!!