Improve performance of block-admin-display-form.html.twig

Last benchmarks from http://drupal.org/node/1898034#comment-7441482

Scenario: admin/structure/block on the homepage, various sidebar blocks.

=== 8.x..8.x compared (519d44ffb3845..519d45a7cd8f5):

ct  : 115,325|115,325|0|0.0%
wt  : 804,456|806,795|2,339|0.3%
cpu : 740,244|740,707|463|0.1%
mu  : 10,287,492|10,287,492|0|0.0%
pmu : 10,643,256|10,643,256|0|0.0%

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=519d44ffb3845&...

=== 8.x..1898034 compared (519d44ffb3845..519d45fd795a4):

ct  : 115,325|125,647|10,322|9.0%
wt  : 804,456|860,084|55,628|6.9%
cpu : 740,244|795,091|54,847|7.4%
mu  : 10,287,492|10,380,288|92,796|0.9%
pmu : 10,643,256|10,823,304|180,048|1.7%

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=519d44ffb3845&...