Resolve "CSV Export for linked row field pointing at table with decimal primary key fails"

Currently the various file exporters convert a linked row by creating a list of the linked table primary field values and exporting that. The values of this list however could just be the raw JSON field list for a table with a file primary key etc. This MR fixes this issue and a related CSV export bug by converting the primary field values to their export format also resulting in linked rows looking like this when exported:

json

    "link_row": [
        "linked_row_1",
        "linked_row_2",
        "unnamed row 3"
    ],
    "decimal_link_row": [
        "1.234",
        "-123.456",
        "unnamed row 3"
    ],
    "file_link_row": [
        [
            {
                "visible_name": "name.txt",
                "url": "http://localhost:8000/media/user_files/test_hash.txt"
            }
        ],
        "unnamed row 2"
    ],

xml

    <link-row>
        <item>linked_row_1</item>
        <item>linked_row_2</item>
        <item>unnamed row 3</item>
    </link-row>
    <decimal-link-row>
        <item>1.234</item>
        <item>-123.456</item>
        <item>unnamed row 3</item>
    </decimal-link-row>
    <file-link-row>
        <item>
            <item>
                <visible_name>name.txt</visible_name>
                <url>http://localhost:8000/media/user_files/test_hash.txt</url>
            </item>
        </item>
        <item>
            unnamed row 2
        </item>
    </file-link-row>

csv

"linked_row_1,linked_row_2,unnamed row 3","1.234,-123.456,unnamed row 3","visible_name=name.txt url=http://localhost:8000/media/user_files/test_hash.txt,unnamed row 2"

Testing Notes

• Make a table with a linked row pointing at another table with a decimal/file/etc primary field. Before this MR it would fail, now it should export correctly.
• Table export for a filled table to all formats still works.
• That the slow test all field conversion test still works (no need to check as it's so slow, just marking as i've ran and confirmed this!)

Closes #496 (closed)