[ticket/11978] Fix width of topic search text field

PHPBB3-11978
Merge pull request #2317 from Elsensee/ticket/11978

[ticket/11978] Fix width of topic search text field

* Elsensee/ticket/11978:
  [ticket/11978] Fix width of topic search text field
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/11978] Fix width of topic search text field