[Kickoff] Reduce the margins of KickoffItem, KickoffHighlight and use smallSpacing

Summary: A lot of space on the left and right of each KickoffItem was wasted, so I've reduced the margins. This improves the readability of long application titles and descriptions. I've also changed the margin units from gritUnit to smallSpacing for KickoffItem and KickoffHighlight, which slightly reduced the margins of KickoffHighlight.

Test Plan:
Before
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Category: {F6755689}
Long description: {F6755688}

After
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Category: {F6755684}
Long description: {F6755687}

Reviewers: #plasma, #vdg, ngraham, hein

Reviewed By: #plasma, #vdg, ngraham, hein

Subscribers: hein, ngraham, filipf, rooty, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19874