Fix HTMLText AbsoluteLinks not parsing placeholder

Add an AbsoluteLinks method to HTMLText that parses placeholders instead of returning the raw value.
Upload: refactor file versioning
Fix: Incorrect calculation of UploadField height (fixes #2862)
Fix: Remove superfluous iframe resize for UploadField (fixes #2863)
ENH making /dev extendable
NEW: Sanitise task name in runTask
Fixing header case
Merge pull request #3380 from silverstripe-rebelalliance/cwp171

NEW: Sanitise task name in runTask
Merge pull request #3383 from dhensby/pulls/fix-send-file-header

Fixing header case in `send_file`
BUG Fix 'undefined index 0'
Merge pull request #3386 from tractorcow/pulls/3.1/abstracttest

BUG Fix 'undefined index 0'
BUG Custom label set in summary_fields config gets overridden
Merge pull request #3387 from halkyon/summary_fields_custom_label2

BUG Custom label set in summary_fields config gets overridden
BUG Reapply fix for cms crashing due to History.js blindly appending slashes to end of url
Merge pull request #3391 from tractorcow/pulls/3.1/fix-history-js

BUG Reapply fix for cms crashing due to History.js blindly appending slashes to end of url
Don't block rewriting .php URLs
Merge pull request #3394 from simonwelsh/stupid-htaccess

Don't block rewriting .php URLs
Fixing inconsistent use of RewriteRule in docs and install.php5

It currently doesn't match the .htaccess that comes with a checkout of
silverstripe-installer.
Merge pull request #3395 from halkyon/rewriterule_consistency

Fixing inconsistent use of RewriteRule in docs and install.php5
NEW: Sanitise task name in runTask