Add modifiers to simplify handling migrations

Some changes need to be done to the serialized data when this tool is used for a migration (e.g. from Plone 4 to Plone 5 or 6) and/or from Archetypes to Dexterity.

To make this easier we could add a checkbox (checked by default) "Modify data for migrations".

If this is checked then some modifiers will run during export.

These could include:

### Drop unused data

Some data that restapi includes is useless for migrations. E.g. `@components`, `next_item`, `previous_item`.

### Drop all Relations

Relations are migrated seperately. haveing them in the data will mess up the site. This is probably easiest done by switching on custom serializers for `IReferenceField` (AT) and `IRelationChoice` and `IRelationList` (DX) that return none.

### Change default-fieldnames (AT to DX):

```python
# Migrate AT to DX
if item.get("expirationDate"):
item["expires"] = item["expirationDate"]
if item.get("effectiveDate"):
item["effective"] = item["effectiveDate"]
if item.get("excludeFromNav"):
item["exclude_from_nav"] = item["excludeFromNav"]
if item.get("subject"):
item["subjects"] = item["subject"]
```

### Fix datetime data

see #10

### Fix issue with AT Text fields

TextField-export in Archetypes: Inspecting the AT-schema and applying a change for all Textfields if the RichtextWidget is not used (which means the field is probably Text in DX and not RichText).

```
# In Archetypes Text is handled the same as RichText
if isinstance(item.get("description", None), dict):
item[fieldname] = item[fieldname]["data"]
if isinstance(item.get('rights', None), dict):
item['rights'] = item['rights']['data']
```

### Fix collection-criteria

Some criteria have changed, e.g.

``` python
query = item.pop("query", [])
for crit in query:
if crit["o"].endswith("relativePath") and crit["v"] == "..":
crit["v"] = "..::1"
if crit["i"] == "portal_type" and crit["o"].endswith("selection.is"):
crit["o"] = "plone.app.querystring.operation.selection.any"
```

### Fix image links and scales

Use the code in https://github.com/collective/collective.migrationhelpers/blob/master/src/collective/migrationhelpers/images.py to fix links to images and make them editable in TinyMCE.

Start by reviewing the migration modifier categories in this issue, the datetime work referenced by #10, and the image-handling code in collective.migrationhelpers/images.py. Determine how the proposed checkbox and each modifier fit the export flow; done means the migration-specific transformations are defined and covered for the listed cases.

Written by the indexing model from the issue text.