What does this implement/fix?

This PR implements new special POST :batchDelete callbacks for /api/groups, /api/domains, /api/clients, and /api/lists. While not being immediately obvious, this is actually acceptable in the REST-world, see e.g., Custom methods | Cloud APIs | Google Cloud for a similar implementation.

While batch-deletion could indeed by very simple in the sense of

DELETE FROM adlist WHERE id IN (:some_list);

this is unfortunately not possible with SQLite3 as the IN () operator lacks support for binding variables. So I ended up implementing the following workaround that uses parameter binding to eliminate any possibility of SQL injections:

1. Start a transaction
2. Create a TEMPORARY table (lives only in memory) called deltable
3. Iterate through the user input and INSERT each row into this table (values are bound to the table)
4. Then run SQL like

   DELETE FROM gravity WHERE adlist_id = (SELECT id FROM adlist WHERE address IN (SELECT item FROM deltable));
   DELETE FROM antigravity WHERE adlist_id = (SELECT id FROM adlist WHERE address IN (SELECT item FROM deltable));
   DELETE FROM adlist WHERE address IN (SELECT item FROM deltable);

5. Delete the temporary table
6. Commit the transaction

If anything fails, we ROLLBACK to undo anything. Surely this SQL will not win a beauty contest due to the involved subqueries but they are so fast and their results are so small that I couldn't be bothered to use JOIN instead.

Addendum:

It is actually more tricky for domains as they are identified in addition by a type, so their temporary database is (item, type) and the SQL looks like:

DELETE FROM domainlist WHERE domain IN (SELECT item FROM deltable WHERE type = 0) AND type = 0;
DELETE FROM domainlist WHERE domain IN (SELECT item FROM deltable WHERE type = 1) AND type = 1;
DELETE FROM domainlist WHERE domain IN (SELECT item FROM deltable WHERE type = 2) AND type = 2;
DELETE FROM domainlist WHERE domain IN (SELECT item FROM deltable WHERE type = 3) AND type = 3;

This PR should still work fine with web on branch development-v6 which does not use :batchDelete. However, to really get the speed benefits, you'll really need to switch web to the accompanying branch (it has the same name). This ensures that your deletions - regardless how many things you delete - will all be done at once.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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