Fix missing mw-ui-button on Login and create page

Don't use modifiers for pages, which uses MW UI nativeley, modifiers
will only be used, when wgUseMediaWikiUIEverywhere is set to true. This
runs into inconsistent use of MWUI on one page.

Follow up: I61bb3c358f755ed9f2153d94b744c1a9da02c456

Bug: 71618
Change-Id: I99d5803818e4c2719f92cab0d9036a1cbc3943ca