Make notification dropdowns consistent

What does this MR do?

Change notification dropdowns to new style. This affects only profile settings and group notifications. Project notification settings have already been handled by !13193 (merged).

Screenshots

Before	After

What are the relevant issue numbers?

#28921 (moved), #35424 (closed)

mentioned in issue #28921 (moved)

restored source branch winh-notification-dropdowns

restored source branch winh-notification-dropdowns

assigned to @cperessini

added 466 commits

• d067b95c...ef8eb3f6 - 465 commits from branch master
• 54b3908c - Make notification dropdowns consistent

Compare with previous version

@winh LGTM. Approving.

approved this merge request

assigned to @winh

assigned to @iamphill

merged

mentioned in commit 11d2ac1f

changed the description

Changed the description after it turned out that group notifications are affected as well (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13557#note_38964740).

mentioned in merge request !13557 (closed)