Afform - fix contact source field & field defaults

[colemanw]

Overview

2 minor Afform bugfixes:

1. Contact source is supposed to take its default from the afform title but was broken
2. Field defaults were saving incorrectly for some fields

Before

Adding a contact to a form would get a blank value for Source.
Fields defaults with numeric keys (e.g. location type id) were saving as integers instead of strings.

After

Source stays in sync with the Afform title unless manually edited.
Fields defaults set correctly, plus added a helpful "Default Value" title to the dropdown:

[civibot]

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[seamuslee001]

Tested this and worked fine