[FIX] web: Kanban quick create: prevent multiple creation

Before this rev., if the user clicked several times on 'Add' (or
pressed several times 'Enter'), on a KanbanQuickCreate record,
multiple records were created.

This rev. makes sure that this doesn't happen anymore by locking
the widget when it is creating a record ('create' RPC that might
be preceeded by an 'onchange' RPC).

Several test cases are added, to mock the different scenarios that
could occur, as they are all slightly different (depending on if
there is an onchange or not and if the user clicks on Add or press
Enter).

There is still a scenario that needs to be handled, which is the
one with an onchange and where the user clicks on 'Add'. This one
is more tricky and will be handled by a future commit.

Task 1878254.