Search improvements

[ScopeyNZ]

This PR:

• Updates the search query type to use a weighted "MultiMatch" and partial matching using "phrase_prefix"
• Uses the elastic provided sort if a search term is used

Currently matches in the addon name are worth 4 times a match in the description which is worth 3 times as much as a match in the readme. I chose some values which seemed "good". If there's other opinions let me know.

In my testing this provided much more intuitive results than default, where searching something like "image gallery" returns colymba/gridfield-bulk-editing-tools in live but much more focused results like sunnysideup/imagegallery_basic with this PR.

I suggest pulling and testing things personally - beware about the time required to build a database of addons though! 😂

Resolves #72 Resolves #53

[dhensby]

looks very nice :)

[robbieaverill]

Deployed to UAT for testing: http://ssaddons-uat.sites.silverstripe.com/