fix multiple issues from user exports config changes

[hughrun]

• improve nginx config
• fix DATA_UPLOAD_MAX_MEMORY_SIZE default not being an int
• translate fallback value in id_to_username template tag
• make location of setting to turn on user exports easier to locate for admins

fixes #3227
fixes #3231
fixes #3232
fixes #3236

[hughrun]

I'm not at all confident about the nginx changes (at the very least there's surely a more succinct way to do this) so advice on that would be welcome.

@mouse-reeve I'm also not entirely confident I'm using blocktrans properly here. I couldn't work out how to use the {% url 'blah' %} tag inside a translated string.

[mouse-reeve]

I found some other translation issues, so I went through and fixed the lot. To your specific question, you can set the url variable using as to assign the output of a template function to a variable: {% url 'settings-imports' as url %}. Then the blocktrans tag should contain as little HTML as possible, so the <br> tag should live outside the translation block, and there shouldn't be line breaks or trailing whitespace, so I added trimmed to remove it:

        <br/>
        {% url 'settings-imports' as url %}
        {% blocktrans trimmed %}
            User exports settings can be changed from <a href="{{ url }}">the Imports page</a> in the Admin dashboard.
        {% endblocktrans %}

[hughrun]

@bookwyrm-social/code-review this is ready for final review.

There is still an issue with the reverse-proxy nginx config however I'm going to log that as a separate issue rather than bog down this PR, as I'm probably not the best person to fix it.

Thanks @dato and @mouse-reeve for your help!