gui: Fix body padding infinitely increasing due to overlapping modals…

… (ref syncthing#9063) (syncthing#9078)

Opening and hiding multiple modals at the same time as well as opening a
modal before fully hiding the previous one can lead to the body padding
infinitely increasing by the scrollbar width each time, with the only
way to fix it being refreshing the GUI.

Therefore, always try to ensure to open and hide multiple modals one by
one, and also that the previous modal has fully been hidden before
proceeding to open the next one. The most common case when this problem
happens is when saving config changes which displays a GUI blocking
modal that overlaps, e.g. with folder or device modals that have not
been hidden yet.

Ref: twbs/bootstrap#3902 (comment)

Signed-off-by: Tomasz Wilczyński <twilczynski@naver.com>