Fix HTMLText AbsoluteLinks not parsing placeholder

Add an AbsoluteLinks method to HTMLText that parses placeholders instead of returning the raw value.
Fixing FormField not setting readonly attribute on setReadonly(true)
Upload: refactor file versioning
Fix: WYSIWYG now returns valid HTML in tables

The WYSIWYG by default creates invalid HTML5. Creating a table and running it through W3C validator returns the following error: The border attribute on the table element is obsolete. Use CSS instead.
API Injector supports nesting
BUG Resolve issue with DirectorTest breaking RequestProcessor
Injector::nest and Injector::unnest are introduced to better support sandboxing of testings.
Injector and Config ::nest and ::unnest support chaining
Test cases for both Injector::nest and Config::nest
Use Config nesting in `SapphireTest::useTestTheme()`

Using the Config nesting functionality to restore variables
FIX Folder Title not being exactly the same as Name field
Backport to 3.0 of PR #3086
Path resolution cleanup
[ref: CWPBUG-158]
MINOR Use composer @stable for PHPUnit

Using stable will allow some packages to be downloaded as zips instead of clones all the time.
Merge pull request #3058 from tractorcow/pulls/injector-stack-tests

API Injector supports nesting
Merge pull request #3085 from tractorcow/pulls/3.1-path-checking

Path resolution cleanup
Changelog note for https://github.com/silverstripe/silverstripe-cms/pull/1013
BUG Fix data query not always joining necessary tables
Fixes #2846
BUG Better checking of existing files
[ref: CWPBUG-158]
BUG Fix Versioned stage not persisting in Session. Fixes #962
BUG Disabled disruptive test case in DirectorTest
API RequestProcessor and VersionedRequestFilter now both correctly implement RequestFilter
Better PHPDoc on RequestFilter and implementations
Merge pull request #2918 from tractorcow/pulls/3.1-requestfilter-versioned-fixes

BUG Fix Versioned stage not persisting in Session
Path resolution cleanup
Backport of #3085 to 3.0
Merge pull request #3019 from tractorcow/pulls/3.1-dataquery-tables

BUG Fix data query not always joining necessary tables
Merge pull request #3101 from tractorcow/pulls/3.1-fileexists-checking

BUG Better checking of existing files
Adds to nextString() method to ConfigStaticManifest

This is used to get the classname out of the tokens, rather than assuming that
the class name is a single T_STRING.