Don't link record in GridField form message

This is no longer allows through Form->sessionMessage() to avoid XSS.
3.0.7 changelog
BUGFIX: Without casting TreeTitle as HTMLText, unescape HTML appears in TreeDropdownField
Increased CacheTest time delay, avoid flickering tests

Setting a cache lifetime of half a second seems to be
unreliable on Travis when just waiting a second,
so let's wait a bit longer.
Merge remote-tracking branch 'origin/3.0' into 3.1
Merge remote-tracking branch 'origin/3.1.0' into 3.1