deposit: add default values for custom fields on deposit form

[tmorrell]

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Description

There isn't currently a way to set defaults for custom fields in the deposit field. This adds a new config variable and implements it in basically the same way as the existing field.

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[utnapischtim]

question: should we copy paste the documentation from APP_RDM_DEPOSIT_FORM_DEFAULTS or should link to APP_RDM_DEPOSIT_FORM_DEFAULTS so that those two doc strings will not get out of sync?

[tmorrell]

I've copy pasted. I'm not sure how one would do a link, such that it would be appropriately pulled into the code. Have any examples?

[utnapischtim]

i have thought of writing something like format of the field follows the structure of APP_RDM_DEPOSIT_FORM_DEFAULTS, please have a look to that documentation. so it is clear that both have the same form.