Redesign the search function

[pablobm]

We have a few open issues regarding the search function, marked with search finding things through our models :

• Be able to search through a STI Field::BelongsTo #1223
• Support Rails's alias_attribute (inside search) #1692
• Searching translated attributes yields "column does not exist" error #1837
• Searching fields on has many associations returns duplicate results #2048
• Allow searchable on multiple inherited classes  #2147
• ERROR : Postgres backed column with a non-deterministic collation #2177
• A self referential belongs_to creates an incorrect SQL query #2249

The common themes I'm seeing here are:

• Better detection of DB structure, supporting STI, edge cases with associations, aliased fields...
• Provide better hooks and documentation about customization.

I think it's time to rethink how the search works. Ideally I would like to see it changing towards these objectives:

I think we should reimplement the search with these objectives in mind:

• A simpler query format. The current one is undocumented and behaves in an unexpected (in my opinion) way. See "Advanced" search #1218 (comment)
• Better at detecting DB structure.
• A clear way to provide your own search if the default is not enough.

Particularly regarding the second point, we may want to lean more on Arel to do the difficult work for us: https://thoughtbot.com/blog/using-arel-to-compose-sql-queries