Modularize Nginx wordpress-site.conf.j2 and add Nginx conf hooks

[fullyint]

Trellis Nginx includes have been limited to one location, inside the primary server block. This PR adds hooks to enable users to include files or inject snippets in other locations (e.g., outside server block). Here is an example of needing to inject a directive in the http context.

The docs in roots/docs#57 will be most helpful for understanding the intent of this PR. Builds on #687

This PR is simpler than it looks. The first commit accounts for most all the PR's additions/deletions, and doesn't change code. It just breaks the WordPress site conf out into separate modules/files. The second commit (Nginx hooks) adds only 13 lines.

The modularization of the Nginx confs is for convenience, to make the sections clear amongst all the jinja templating complexity. This is the approach already in use for roles/wordpress-setup/templates/https.conf.j2.

The one change made in the modularization is that this PR moves the base WordPress conf out of the nginx role into the wordpress-setup role. This changes this base WP conf from an include whose content is general to all sites, to content injected into each site's conf such that it can be customized per site, via the nginx_location hook.

[fullyint]

I rebased to resolve merge conflicts that arose from the recently merged #692. Both PRs involve(d) caching stuff in Nginx confs.

I used the rebase as an opportunity to add a few tags: nginx-includes on tasks involving the new nginx hooks.

[nathanielks]

@phil was about to create a PR adding some support for trellis living behind a server that does SSL termination, but I remembered this existed and was curious where we're at with this getting merged?

[nathanielks]

Nevermind! I was able to do it pretty simply.

[fullyint]

closed and replaced by the better solution in #740