Enable link to existing ERC if no ERC is created in OJS

[tomniers]

• if no ERC is created in OJS there should be nevertheless the function to link an existing ERC with the current submission
• there is a need for an input field where the user can indicate the ErcId of the existing ERC which should be linked with the current submission
• Take care that this can be only done if there is no ERC created with the current submission. Otherwise there could be two ERCs for one submission and that should not be the case.