Disable user exports

[hughrun]

• add setting to enable and disable user exports (defaults to False)
• do not allow user exports when using s3 storage
• do not serve non-image files from /images/ (requires admins to update nginx settings)
• increase default file upload limit to 100MB to enable user exports to be imported (can be changed in .env)

Fixes #3210

[prolibre]

Hello @hughrun It's me again
sorry, I don't mean to pester you :-)

I think there's a new variable missing in .env.example to allow (or not) export.
The variable that would correspond to user_exports_enabled

[hughrun]

@prolibre it's a new value in the site settings model. You need to run ./bw-dev migrate or for dockerless I think it's python manage.py migrate

[prolibre]

@hughrun
I misspoke (because of the translation).
What variable should I put in my .env file (I don't use docker) to activate the export ? I use the .env file as a configuration file.

[hughrun]

My apologies @prolibre this has not been explained properly. You do not use the .env - there is a setting in the Admin panel. Go to /settings/dashboard and then click on "Imports" in the menu. That shows various settings for book and user imports, and also now has a setting for enabling or disabling user exports. I will add an Issue to make this a bit clearer.