add support for custom sources

[i-just]

Description

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

Related issues

#1488