fix(installer): database page lost its Username and Password boxes

Reported from the Server 2019 test: the Existing database page rendered a
truncated "Username:" label and no input boxes at all below it, so there was no
way to enter connection details.

CreateInputQueryPage lays its fields out BELOW the description text. The
description had grown to include a five-line CREATE DATABASE / CREATE USER /
GRANT block, added so a DBA could be handed the exact SQL. With five fields
underneath, the last two fell past the bottom of the page surface, where they
are simply not drawn - no error, no scrollbar, just missing controls.

The description is back to three lines. The SQL moves to
docs/INSTALL-WINDOWS.md, which is where someone would look for it anyway and
where it can be copied without being retyped from a wizard page.

Wizard page descriptions are a fixed budget: anything long enough to be worth
reading twice belongs in the guide, not on the page.

deploy/windows/installer/ShopDBFlask.iss

@@ -408,13 +408,9 @@ begin
 
   DbDetailsPage := CreateInputQueryPage(DbChoicePage.ID,
     'Existing database', 'Connection details',
-    'The database and application user must already exist. Your DBA creates ' +
-    'them; the installer does not, so that it never needs administrative ' +
-    'rights on your database server. If they need the exact statements:' + #13#10 +
-    '    CREATE DATABASE shopdb_flask CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;' + #13#10 +
-    '    CREATE USER ''shopdb''@''%'' IDENTIFIED BY ''<password>'';' + #13#10 +
-    '    GRANT ALL PRIVILEGES ON shopdb_flask.* TO ''shopdb''@''%'';' + #13#10 +
-    'The utf8mb4 charset matters - the default on older servers is latin1.');
+    'The database and application user must already exist - the installer does ' +
+    'not create them, so it never needs administrative rights on your database ' +
+    'server. The exact SQL to give your DBA is in the INSTALL-WINDOWS guide.');
   DbDetailsPage.Add('Host:',     False);
   DbDetailsPage.Add('Port:',     False);
   DbDetailsPage.Add('Database:', False);