Request features from Review section with one call

[dpordomingo]

required by #198 and https://github.com/src-d/backlog/issues/1202
fixes #229

As defined by #198 (comment), the ML Feature Extractor API will take a couple of UAST, and it will return a bunch of features per each UAST, and a score as a pair-feature

This PR prepares the frontend to do one call per FilePair being reviewed to obtain the features of each file, and the score itself.
This PR prepares the backend to answer following the expected schema.

No UI changes were done.

It was also added a
3445eb0 commit to fix #229 because otherwise the FilePairs beyond the first one cannot be reviewed.

Pending work

To be done once the ML Feature Extractor API is ready, but that does not block this PR:

• the new API musth be called from the features handler (pointed by the TODO),
• the model.Features must be removed,
• the features db table must be removed → running features-drop-table migration from DB Migrations #221

[carlosms]

👍

[smacker]

I didn't get why we need to change it to 1 call now.
As I understand there is no API yet. And we can actually use https://github.com/src-d/gemini/blob/master/src/main/python/server.py as such api. It accepts 1 uast + params for extraction and returns list of features.

I would better extract fix for #229 in a separate commit and wait with other changes as api isn't ready yet.

on another hand changes are small, so if you want to merge it, it's okay.

[dpordomingo]

@smacker all your doubts are properly explained in the PR description; for example:

• we need to call the API with two UAST bec it will return the similarity score between them,

[...] the ML Feat. Extr. API will take a couple of UAST,and it will return [...] a score as a pair-feature

[bzz]

Although #233 (comment) has the point, the explanation about necessity of this work holds and even if it's a bit preliminary, it LGTM.