Views for metrics v2-v3

[antoinehashbang]

https://phabricator.endlessm.com/T32156

[liZe]

@wjt As discussed earlier, this branch is not ready to be merged, but I’d appreciate a "quick" review to check that everything is OK so far.

What’s missing and/or wrong:

• ~conflict~ (fixed),
• tests and lint,
• strange commits, rebase required,
• migration including V2+V3 views is not included,
• union_all in queries requires columns, not tables,
• channel_id needs to replace channel columns, with a solution to get both V2 and V3 channels.

[liZe]

I wish I had some extra time to finally merge this PR…

I’ve marked as "resolved" the comments of @wjt’s review that are now resolved. Here’s what’s missing:

• using channel_id instead of the channel attributes;
• renaming views;
• adding aggregate events;
• (optional) overriding View.__getattr__ to wrap more table attributes;
• and finally, rebasing commits to get a nice history.

Don’t hesitate to ping me in this PR if you need anything!