[FFe needed] Update NGINX in Cosmic to 1.15.3 for bugfixes

There are a number of bugs fixed in the HTTP DAV module shipped with nginx by upstream as part of NGINX 1.15.3, and a workaround to an HTTP/2 bug. Given that for Cosmic we are tracking NGINX Mainline, the simplest solution for this is a

The following is the changelog for NGINX 1.15.3:

Changes with nginx 1.15.3 28 Aug 2018

    *) Feature: now TLSv1.3 can be used with BoringSSL.

    *) Feature: the "ssl_early_data" directive, currently available with
       BoringSSL.

    *) Feature: the "keepalive_timeout" and "keepalive_requests" directives
       in the "upstream" block.

    *) Bugfix: the ngx_http_dav_module did not truncate destination file
       when copying a file over an existing one with the COPY method.

    *) Bugfix: the ngx_http_dav_module used zero access rights on the
       destination file and did not preserve file modification time when
       moving a file between different file systems with the MOVE method.

    *) Bugfix: the ngx_http_dav_module used default access rights when
       copying a file with the COPY method.

    *) Workaround: some clients might not work when using HTTP/2; the bug
       had appeared in 1.13.5.

    *) Bugfix: nginx could not be built with LibreSSL 2.8.0.

The only 'Feature' change here of relevance is that "keepalive_timeout" and "keepalive_requests" can be provided in the 'upstream' block of a config, but functionally there are no other changes.

The other two feature changes are BoringSSL-specific, and not relevant for Cosmic as we use OpenSSL for ouir builds.

The remaining bugfixes should be included to fix issues with the HTTP DAV module.

The regression risk of this is low, as these bugfixes originated upstream.

Package builds for this are in-progress at a PPA specifically for this with all buildable arches enabled - https://launchpad.net/~teward/+archive/ubuntu/nginx-1790149/+packages

Installation of the packages seems to work fine on my local builds (however, they were built via my automated build environment which does not currently have the build logs published, only the result, please refer to build logs in the PPA for build logs)