Added sorting by relevance (_score)

[giocalitri]

Fixes #868

[pwilkins]

As @giocalitri warned, this may get complicated.

Setting the default sort order to "relevance" moves the learning resource titled "Astronauts playing catch" to the top of the result list, but adding "Astronauts playing catch" to the description of another LR bumps the titled LR to second spot.

Since I'm doing this locally, the fact that there are no LR descriptions (other than the one I added) may have unbalanced its relevance.

[giocalitri]

@pwilkins was the search string "Astronauts playing catch" ? Can you check the score of the two learning resources: it is in the JSON returned by search API

[noisecapella]

Code looks good, testing functionality now

[noisecapella]

I was just wondering, shouldn't it be descending relevance since the most relevant item should be first?

[giocalitri]

it actually is: I will remove the string (just relevance)

[noisecapella]

Sounds good, feel free to merge after :+1:

[giocalitri]

@noisecapella done: will merge after tests pass again

[giocalitri]

Travis finally worked: merging