Fix: Enable delete button for users with canDelete() but no canEdit() permissions

$form->makeReadonly(); iterates over all fields/actions, disabling the 'delete' button even when the user has permission
ENHANCEMENT: allow disabling of Versioned prepopulate_versionnumber_cache

In large sites this can take a very long time, drastically slowing down the CMS
admin.  Even though the versions will then need to be queried individually,
this is still significantly faster than loading hundreds of thousands of
version numbers in one query and populating the cache array.
BUG Fix JS issues in treedropdownfield for link editor
Merge pull request #2086 from robert-h-curry/1954-link-editor

BUG Fix JS issues in treedropdownfield for link editor
Merge pull request #2062 from jthomerson/enhancement_3.1_allow_disabling_prepopulate_version_number_cache

ENHANCEMENT: allow disabling of Versioned prepopulate_versionnumber_cache
Merge pull request #2051 from kinglozzer/candelete-gridfield-fix

Fix: Enable delete button for users with canDelete() but no canEdit() permissions
Merge pull request #2088 from g4b0/cache-comments

Inline documentation updated
API Allow $summary_fields to support methods on DBFields
AssetAdmin reset button and chosen.js (fixes #1640)

See https://github.com/harvesthq/chosen/issues/215
Merge pull request #2087 from tractorcow/3.1-summary-field

API Allow $summary_fields to support methods on DBFields
Force TreeDropdown to remember selected title.
Merge pull request #2092 from robert-h-curry/tree-dropdown-unsticky-selection

Force TreeDropdown to remember selected title.
Always load preview when in preview mode, not in edit mode
API New CMSForm class to allow validation responses in CMS (fixes #1777)

Thanks to @willmorgan for getting this discussion started
(see https://github.com/silverstripe/sapphire/pull/1814).
Consistent Form setters (returning $this on setHTMLID())