Added bottom margin to prevent overlapping buttons

[iainhallam]

Fixes #1954

When using a plugin such as https://www.dokuwiki.org/plugin:include, you can end up with several empty sections each of which has an edit button. In this case, because the buttons have been given a -1.4em top margin, they effectively take up no space, so Chrome and Firefox (not tested in IE) overlap them, one on top of each other. By adding back a 1.4em bottom margin to each secedit block, the floats now take up space again and will not get in each other's way.

[mention-bot]

@iainhallam, thanks for your PR! By analyzing the history of the files in this pull request, we identified @selfthinker and @MrClow to be potential reviewers.