Adds support for importing into the relation fields that have custom source(s) selected.

assets field => you can't select a custom source as the field's source
categories field => the field can only have one source selected; it can be a custom source; in this PR I have:
- disabled creating new categories if a custom source is selected (because of ensuring all the condition rules are met); this includes removal of the “Create categories if they do not exist” from the mapping screen;
- made it possible to import existing categories into a categories field which has custom source selected
entries field => the field can have multiple sources selected, including custom ones; in this PR I have:
- made it possible to import entries into an entries field that has a custom source selected
- disabled creating new entries if the only selected sources are custom ones (because of ensuring all the condition rules are met); this includes removal of the “Create entries if they do not exist” from the mapping screen;
tags field => n/a; there is no area to create custom sources for tags
users field => same as with entries

craftcms / feed-me

1488