Hardening Improvements - security header recommendations

Some recommendations from Sucuri:

- Missing security header for ClickJacking Protection. Alternatively, you can use Content-Security-Policy: frame-ancestors 'none'.
- https://docs.sucuri.net/warnings/hardening/security-headers-x-frame-options/
- You can enable it by modifying your Apache settings or your `.htaccess` file - on server side
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors

- Missing security header to prevent Content Type sniffing.
- https://docs.sucuri.net/warnings/hardening/security-headers-x-content-type-nosniff/
- You can enable it by modifying your Apache settings or your `.htaccess` file - on server side

- Missing Strict-Transport-Security security header
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security
- applicable only to HTTPS sites on Laddr, can it be configurable per Emergence instance/site?
- should be done on server side

- Missing Content-Security-Policy directive.
- https://blog.sucuri.net/2018/04/content-security-policy.html
- We recommend to add the following CSP directives (you can use default-src if all values are the same): script-src, object-src, base-uri, frame-src
- try to block executing scripts added in content (i.e. page, buzz, project description, comment etc)
- Use "report-uri" to log failed requests. Endpoint to send report json to: https://report-uri.com/#prices (free up to 10.000 requests per month); when testing in production use "report-only" to send reports to URL endpoint what would be blocked by set CSP rules.

- Leaked PHP version. Your site is displaying your PHP version in the HTTP headers. Please set expose_php = Off.
- https://secure.php.net/manual/en/ini.core.php
- this should be done on server side (Emergence hosting)

Check full report at:
https://sitecheck.sucuri.net/results/codeforphilly.org (same results are for other Laddr instances)

まず、issue で言及されている Apache 設定または .htaccess 設定を確認し、どのセキュリティヘッダーが Emergence ホスティングによって制御され、どれが Laddr インスタンスによって制御されるのかを判断します。HTTPS サイトがどのように設定されているか、また PHP がそのバージョンを公開しているかを確認します。既存のサイトを壊すことなく、該当するヘッダーと PHP 設定が有効になっていれば完了です。

索引モデルが issue の本文から書いたものです。