FIX Requirement documentation of MySQL driver

Currently the documentation states that the `mysql` module for PHP is required, however as of #84 (over 2 years ago) this is no longer the case and the required module is now `mysqli`.
Merge pull request #2848 from nedmas/patch-2

Requirement documentation of MySQL driver
FIX: Searchable fields with dot notation can be inherited from summary_fields (fixes #1429)
BUG: Director::test() don't respect port settings in $_FILE_TO_URL_MAPPING

Director::test() don't set the HTTP_HOST with the port number if that has been set.
Later call to Director::makeRelative() will return wrong value because of the strict string matching
(http://localhost/ != http://localhost:8000)

This bug affects all modules that are using Director::test in CLI where the $_FILE_TO_URL_MAPPING
have been set to use a domain with a port in it, i.e. static publishers.
Merge pull request #2852 from stojg/director-test-http-port

BUG: Director::test() don't respect port settings in $_FILE_TO_URL_MAPPING
Merge pull request #2845 from kinglozzer/1429-summary-searchable-fields

FIX: searchable_fields inheritance from summary_fields includes methods called on fields (fixes #1429)
Preliminary 3.1.3 changelog
Updated translations
NEW Allow specifying element whitelist in HTMLText fields
Merge pull request #2853 from hafriedlander/feature/htmltext_whitelist

NEW Allow specifying element whitelist in HTMLText fields
Note about SiteTree.ExtraMeta in security docs
Merge remote-tracking branch 'origin/3.1'