This fixes #6393 by modifying the constraint to use an IN condition
which supports both Postgres and SQL Server. Previously, the `|` (OR)
condition was only supported by Postgres because SQL Server only
supports AND conditions.
The implementation follows suggestions from @gasman in
https://github.com/wagtail/wagtail/issues/6393#issuecomment-732161057:
* Migration 0050 is modified to not break on SQL Server
* Added migration 0060 to add or replace the constraint
Additionally, this allows for and documents a `DATABASE_DRIVER` env
variable to be set for testing, to allow a different SQL Server driver
(e.g. FreeTDS on Mac/Linux); and adds the specific `host_is_server`
option for FreeTDS (won't affect SQL Server Native Client on CI).
Facebook and Instagram embeds configuration section should include the default finders for all other providers. It's unclear for someone who just wants to fix his facebook embeds
Same approach as 4023a90d6e for images - define an UploadedDocument model to hold the uploaded file up to the point where the required metadata has been supplied.
Boto3 has been around for a long time now. Adding a Boto-specific parameter in a tutorial is confusing (it was for me) as people are likely to install Boto3 directly and skip Boto. The `AWS_HEADERS` parameter was replaced by the `AWS_S3_OBJECT_PARAMETERS` parameter in Boto3 and the parameter that controls the access control list specifically is the `ACL`.
By the way, are the instructions on line 50 still relevant today?
Signal receivers must accept `**kwargs`, otherwise one runs into this:
```
File "/lib/python3.8/site-packages/django/dispatch/dispatcher.py", line 94, in connect
raise ValueError("Signal receivers must accept keyword arguments (**kwargs).")
ValueError: Signal receivers must accept keyword arguments (**kwargs).
```
