Introduce new /$DB/-/query endpoint, soft replaces /$DB?sql=...

[asg017]

refs #2360

This PR adds a new dedicated endpoint to querying a database in Datasette with SQL. Previously, one would use the /dbname?sql=select... convention to do this, but it shared the same endpoints as normal database. So this PR adds a dedicated /dbname/-/query?sql=... route specifically for this task.

Old URLs that use the previous /dbname?sql=... endpoint will still work - we internally just redirect that to the new URL.

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📚 Documentation preview 📚: https://datasette--2363.org.readthedocs.build/en/2363/

[simonw]

Spotted that the form on this page:

Still targets /content which then redirects. I'll push a fix for that.

datasette/datasette/templates/database.html

Line 24 in da1eb18

<form class="sql" action="{{ urls.database(database) }}" method="get">

[simonw]

That test failure was the same intermittent failure we've seen on 3.8 in a few other places:

• Intermittent test_delete_row test failure  #1919