Add db migration

[bobholt]

This is a pre-requisite for adding a column to the pull_request table to fix the multi-comment situation (and for future database changes we want to perform on a live server).

Note this includes the standalone certbot versioning fix (https://github.com/w3c/wptdash/pull/18) as a different commit SHA. If that lands, this should be rebased.

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This change is 

[jgraham]

Reviewed 8 of 8 files at r1. Review status: all files reviewed at latest revision, 3 unresolved discussions.

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migrations/env.py, line 1 at r1 (raw file):

from __future__ import with_statement

Surely this isn't needed here?

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migrations/env.py, line 17 at r1 (raw file):

# add your model's MetaData object here
# for 'autogenerate' support

Let's remove some of the commented out copy-paste (and maybe make the comments more relevant).

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migrations/README, line 1 at r1 (raw file):

Generic single-database configuration.

This doesn't seem too helpful. Can we link to wherever this configuration came from?

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Comments from Reviewable

[bobholt]

Review status: all files reviewed at latest revision, 3 unresolved discussions.

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migrations/env.py, line 1 at r1 (raw file):

Previously, jgraham wrote…

Surely this isn't needed here?

This file was auto-generated by flask-migrate, so I didn't even look at it. I don't believe it will by any other automated scripts, so I'll clean it up.

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migrations/README, line 1 at r1 (raw file):

Previously, jgraham wrote…

This doesn't seem too helpful. Can we link to wherever this configuration came from?

Another auto-generated file. I can remove it or add a note that all files in this directory are auto-generated by the migration script.

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Comments from Reviewable

[jgraham]

Review status: all files reviewed at latest revision, 3 unresolved discussions.

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migrations/README, line 1 at r1 (raw file):

Previously, bobholt (Bob Holt) wrote…

Another auto-generated file. I can remove it or add a note that all files in this directory are auto-generated by the migration script.

I assume the idea is that one cleans this up. But either way is fine.

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Comments from Reviewable

[bobholt]

Removed the README and cleaned up some comments. Remaining comments seem useful to me.

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Review status: all files reviewed at latest revision, 3 unresolved discussions.

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migrations/env.py, line 17 at r1 (raw file):

Previously, jgraham wrote…

Let's remove some of the commented out copy-paste (and maybe make the comments more relevant).

Done.

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migrations/README, line 1 at r1 (raw file):

Previously, jgraham wrote…

I assume the idea is that one cleans this up. But either way is fine.

Done.

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Comments from Reviewable

[jgraham]

Reviewed 3 of 3 files at r2. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable