[FIX] web: database selector and manager

The database manager and selector triggered an internal server error
according to the modules installed.

The issue is that some modules (like utm) overrode ir_http's dispatch
and openned a cursor before calling super. Following rev[1], the
database manager can tell you the incompatible databases. For that, it
opens a new cursor to each database and get back the Odoo's version.
After the information is fetched, the cursor is closed. Closing a cursor
does not release it from the connection pool, so the database cannot be
dropped and here comes the fix [2]: this commit removes all cursors to
`database` from the connection pool.

Now, if a module overrides the ir_http's dispatch and open a cursor
before calling `super`, the automatic commit/close done at the end of
a successful request cannot succeed in the database manager or
selector as the cursor is closed during the call to super, leading to
an internal server error.

To workaround this issue, we explicitely set the cursor to None, as it
will prevent the automatic commit/close at the end of the request.

This is similar to rev 5c6087e38e

[1] 96f703ff84
[2] 610036756a

addons/web/controllers/main.py

@@ -639,10 +639,12 @@ class Database(http.Controller):
 
     @http.route('/web/database/selector', type='http', auth="none")
     def selector(self, **kw):
+        request._cr = None
         return self._render_template(manage=False)
 
     @http.route('/web/database/manager', type='http', auth="none")
     def manager(self, **kw):
+        request._cr = None
         return self._render_template()
 
     @http.route('/web/database/create', type='http', auth="none", methods=['POST'], csrf=False)