Implement search options to enable more performant queries

[FabriceCastel]

Address issue: #2745

Change

Add the following config options to better control how administrate generates search queries targeting a given field:

• search_exact
  • true: the search will be performed using an equality comparison
  • false (default): the search will be performed using a substring search as ... LIKE '%your-search-term%'
• search_lower
  • true (default): search terms and target columns are lowercased
  • false: search terms and target columns retain their original casing

This change also introduces search_requires_string_cast? on Field classes to avoid casting search columns with CAST(... AS CHAR(256)) when the casting is redundant (ie. for string-like columns).

The defaults for all of the above have been set so as to retain existing behaviour by default.

Usage/Rationale

• search_exact allows users to support exact searches (for eg. an email address, a phone number, a postal code), in cases where substring matching isn't applicable and/or incurs too heavy a performance penalty on the db side when dealing with fairly large data sets
• search_lower controls whether or not we lowercase the term and column being compared; this both adds support for case-sensitive searching as well as allows folks to disable the LOWER(...) clause in the generated query to avoid having to support it with extraneous db indices (eg. when searching for a phone number or zip code)

[shadoath]

These are great additions for querying large datasets!

[nickcharlton]

Great, thanks for opening this! Could you make sure every change has a corresponding test?