nginx: simplify enketo passthrough rewrites

[alxndrsn]

Noted while working on #987

What has been done to verify that this works as intended?

Current tests continue to pass.

Why is this the best possible solution? Were any other approaches considered?

Using rewrite ... last and separating the two blocks means that passthrough URLs are now rewritten, and the entire config file re-evaluated for the new URLs:

stops processing the current set of ngx_http_rewrite_module directives and starts a search for a new location matching the changed URI;
- https://nginx.org/en/docs/http/ngx_http_rewrite_module.html#rewrite

This is in contrast to the current approach using rewrite ... break, which:

stops processing the current set of ngx_http_rewrite_module directives as with the break directive;
- https://nginx.org/en/docs/http/ngx_http_rewrite_module.html#rewrite

and the break directive is defined as:

If a directive is specified inside the location, further processing of the request continues in this location.

Which means that break, unintuitively, allows continuing of the current location block.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced