Use bootstrap input-group for date range

[jhawthorn]

Since #1782, the "date range" fields have looked off because they had a hardcoded size.

This PR groups the two inputs and the separator together inside a bootstrap input-group

This changes all uses of this pattern throughout the admin.

The line-height of the separator is slightly off under our current bootstrap, this is fixed by #1816 (used in screenshot above).

[tvdeyen]

👍 super.

this really could be a partial, right?

[tvdeyen]

Interesting, I have a double border on the addon

Also the line height seems to be off. Fixed it with:

.input-group-addon:not(:first-child):not(:last-child) {
  border-left: 0 none;
  border-right: 0 none;
  line-height: $line-height;
}

Also fixed by #1816 (which is merged)