No validation for unique dashboard code

[owlet1997]

Enviroment:

Jmix version: 1.0.0-SNAPSHOT
Jmix Studio plugin version: 1.0.SNAPSHOT2839-211
IntelliJ version: IntelliJ IDEA 2021.1 (Community Edition)

TC:

1. Open attached project -> run app
2. Open Dashboard -> Dashboards -> Create new
3. Set name chartsw, code w -> Save
4. Create new -> Set name chartsw, code w -> Save

AR:

dashboard successfully created

ER:

according to documentation dashboards should have unique codes so there should be notification about duplicate codes

[Desire456]

Commit reference 5cc4912

[Desire456]

For QA: don't test it on hsqldb