Django-tables2

- Another round of template cleanup.
- Fresh screenshots
- Prevent crash in `RelatedLinkColumn` for records without `get_absolute_url()`.
- Raise `ValueError` when `Table.Meta.model != QuerySet.Model`.
- Raise `TypeError` when incorrect types are used for `Table.Meta` attributes (fixes [517](https://github.com/jieter/django-tables2/issues/517))
- Fix: `Table.Meta.sequence` with `extra_columns` can leads to `KeyError` (fixes [486](https://github.com/jieter/django-tables2/issues/486))

- Fixed translation of 'previous' for some languages (fixes [563](https://github.com/jieter/django-tables2/issues/563))

- Cleaned up templates to add consistency in what is presented across all templates.
- Added bootstrap4.html template
- Fixed translation inconsistencies.

- Moved table instantiation from `get_context_data` to `get_tables` [554](https://github.com/jieter/django-tables2/pull/554) by [sdolemelipone](https://github.com/sdolemelipone)
- Pass request as kwarg to `template.render()`, rather than as part of context. (fixes [552](https://github.com/jieter/django-tables2/issues/552))

- Do not perform extra `COUNT()` queries for non-paginated tables. Fixes [551](https://github.com/jieter/django-tables2/issues/551)

- Add new method `paginated_rows` to `Table` to replace fallback to non-paginated rows in templates.
- Prevent mutation of the template context `{% render_table %}` is called from (fixes [547](https://github.com/jieter/django-tables2/issues/547))
**Possible breaking change**: the context variables of the template `{% render_table %}` is called from is no longer available in the table's template. The `table` variable has an attribute `context`, which is the context of the calling template. Use `{{ table.context.variable }}` instead of `{{ variable }}`.