add island labels (#399, #646, #703)

[nvkelso]

Connects with #399 to add island, islet & other labels. Also refactors how the earth layer is constructed.

• [query] add earth queary JINJA
• [query] hook up schema prefunction for new earth layer
• [query] hook up create sql functions and sql.jinja to new earth layer
• [filter] remove continent place labels from places layer
• [filter] init earth YAML filter, add continent place label to earth layer
• [filter] adjust island, islet zoom ranges
• [apply] hook up data/apply functions, set new fields for new earth layer
• [tests] add full set of island label tests
• [migration] add custom migration for new earth layer features
• [docs] add new kinds to earth layer docs

[nvkelso]

@zerebubuth This is still a WIP, but it's pretty close to done. Please have a look around, particularly at migration and query parts.

I've verified the zooms are working well in the DB for the test cases, but still need to fully run the test via python.

[nvkelso]

Also connects to #703 to move the continent label placement from places layer to earth layer.

[nvkelso]

Also connects to #646 to move the last layer, earth, to the new YAML setup.

[zerebubuth]

I think we prefer to do the addition of extra parameters (label_placement, boundary) via YAML now, to avoid having to do NULL AS ... in the other queries.

[nvkelso]

Good point, I'll revise.

(This is loosely based off the water.jinja2 file, probably similar cleanup over there, but I'd rather not mess with that in this PR.)

[nvkelso]

I've got it all working (!?) except for these handful of tests which fail:

• Failing tests: 98cc13d

Similar test do pass, though, and the features exist in the DB and seem to be marked with the expected zoom values.

The synthetic_columns: - test bit should also be investigated & resolved.

Because I'll be traveling the rest of the week, please pick up this PR and see it to completion. Thanks!

[zerebubuth]

Tests pass for me. I think all the issues above should be fixed now. @rmarianski could you review this, please?

[rmarianski]

👍