Changes to support postgresql

[lexicalunit]

When I started testing using PostgreSQL as the backend it became obvious quickly that some changes would need to be made to accommodate it.

• The database adapter for PostgreSQL, psycopg2, needed to be added.
• The GROUP BY query in sqlite doesn't work in a real database like PostgreSQL, so I re-wrote it in such a way that it will work on both sqlite and real databases.
• Since sqlite treats all integral data types the same, it didn't matter if we used INTEGER or BIGINT for the author columns. However in any real database you will need BIGINT for these. So I added a migration to alter those columns. This doesn't actually affect the behavior of sqlite at all, but it makes things work on PostgreSQL.
• If you're using PostgreSQL for tests you have to ensure that the tests have a clean slate before each test. You got this for free with sqlite so I had to write some new code to ensure this happens.
• For real databases you also need to make sure you are properly closing connections in the test suite.
• Because sqlite completely ignores FK constraints, I needed to update some of the Figures tests to ensure that those constrains were actually satisfied for them to pass on PostgreSQL.

I also added a script and some documentation to help support future database migrations.

TODO:

• [x] Add a CI matrix to run tests against both sqlite and postgresql.
• [x] Resolves https://github.com/theastropath/turbot/issues/174

[codecov-io]

Codecov Report

Merging #173 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #173   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         6    +2     
  Lines          214       265   +51     
  Branches        12        12           
=========================================
+ Hits           214       265   +51     

Impacted Files	Coverage Δ
src/turbot/data.py	100.00% <100.00%> (ø)
...0279af35_keep_track_of_authorized_channels_per_.py	100.00% <100.00%> (ø)
...versions/9b107d322c46_use_bigint_for_author_ids.py	100.00% <100.00%> (ø)

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[lexicalunit]

Added an !authorize command so that different servers can authorized their own set of channels instead of only being able to set it on the command line.

[lexicalunit]

Updated to resolve https://github.com/theastropath/turbot/issues/174.