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