Handle get visualization request from frontend

[nartydev]

Create an handler to compute a visualization for on test.

It must handle:

• simple data retrieval
• precomputing of meaningful stats (average, percentile,...)
• precomputing visualization data (see frontend lib)
• precomputing of comparison of 2 tests
• filtering responses within a test

Note: the client must provide an auth token (Authorization header).

Note: all the features to support may be split into individual handlers/endpoints.

Note: from the brainstorming session :

[moreirathomas]

It must handle:

• simple data retrieval
• ~precomputing of meaningful stats (average, percentile,...)~
• ~precomputing visualization data (see frontend lib)~
• ~precomputing of comparison of 2 tests~
• filtering responses within a test

All precomputations concerned with one test are done inside a worker. Precomputation of comparison between tests is done by the client (most of the raw data would have been processed, don't deal with that here).