set width of inputfield to a definde value

What does this MR do?

Set the width of the inputfields in stores to 50% with min-width, big enough to fit samll screens (e.g. mobilephone)

How confident are you it won't break things if deployed?

100%

Links to related issues

https://yunity.slack.com/archives/C1T1ZM9A4/p1552333257150500

Screenshot

collapses as it should

Heres the actual version:

And the text in the field wich could be choosen is shortened a bit:

Checklist

• added a screenshot from localhost
• no unrelated changes (just removed the useless px at .ui-progressbar)
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))