Getting started on translations

[mistercrunch]

This PR gets things started to enable translations of Caravel.

By default only english is enabled. As we get better completeness for certain languages, people can enable these languages in their environment, by configuration. See the new Tranlations section in CONTRIBUTING.md

[landscape-bot]

Repository health decreased by 0.54% when pulling bd13dd9 on mistercrunch:translate into d7ea473 on airbnb:master.

• 3 new problems were found (including 1 error and 2 code smells).
• No problems were fixed.

[coveralls]

Coverage increased (+0.007%) to 80.974% when pulling bd13dd94c66e7c6213ae5c102eaf3e6cd97a84ac on mistercrunch:translate into d7ea473 on airbnb:master.

[landscape-bot]

Repository health decreased by 0.54% when pulling f5c31b5 on mistercrunch:translate into d7ea473 on airbnb:master.

• 3 new problems were found (including 1 error and 2 code smells).
• No problems were fixed.

[coveralls]

Coverage increased (+0.007%) to 80.974% when pulling f5c31b5f0705d42bd0866a7716c6a1160f560085 on mistercrunch:translate into d7ea473 on airbnb:master.

[coveralls]

Coverage increased (+0.007%) to 80.869% when pulling e8471d4 on mistercrunch:translate into ec7dbed on airbnb:master.

[landscape-bot]

Repository health decreased by 0.14% when pulling e8471d4 on mistercrunch:translate into ec7dbed on airbnb:master.

• 2 new problems were found (including 0 errors and 2 code smells).
• 1 problem was fixed (including 0 errors and 1 code smell).

[aibati2008]

@mistercrunch hi, I tried to translate the language to Chinese which described in CONTRIBUTING.md. But it shows the below error while execute the command "pybabel init -i ./babel/messages.pot -d caravel/translations -l zh".

and the file messages.pot is not exist under the babel folder indeed, please advise how to do. Thank you!