Jumbotron: remove the hard-coded font-size from jumbotron.less and moved it into variables.less where other .font-size-xxxxx things now live.

I probably missed a few things and screwed up a few others, but this
takes the place of much of #9910. I haven't added many variables—that
should be reserved for v3.1—but I have rejiggered the layout, added
missing ones, corrected wrong values, etc.

/cc @cvrebert @juthilo @saas786

IMHO this default is better

Deprecate container-{device} vars for container-{size} classes; updates customizer and grid to do the same

iOS doesn't pick up the z-indexing properly on the dropdowns unless you
specify one for the parent navbar.

Refactor alerts to have a base class, but no default class, thus requiring a modifier class at all times

Refactor panels to have a base class for improved flexibility when customizing (and no overriding to start, too)

New variable names were missing here and there.

* Extract color styles from .navbar { } and put them into
.navbar-default { }
* Change variable names
* Change the docs accordingly (i.e., add the .navbar-default class to
all default navbars)
* Change customizer accordingly

improve pagination's active state so it looks less like the active state and more like active nav items

Grid classes have been changed to account for a fourth set of classes,
meaning we now have XS, S, M, and L options. Specifically, we made the
following changes:

* Renames `.col-` to `.col-xs-`
* Adds `.col-md-` for devices 992px wide and up
* Remaps `.col-lg-` for devices 1200px wide and up

Alongside that, we've updated our media queries to better handle
advanced grid layouts:

* All column classes now kick in when they hit a `min-width`, undoing
the #9128 fix in
https://github.com/twbs/bootstrap/commit/db45a60cc86190a060f0baf0b3961cb
c690f3a65
* All column offsets, pushes, and pulls are scoped to an appropriate
range, meaning a `min-width` and `max-width` (thanks for the idea,
@eratzlaff)

We've also modified the widths of our grid containers for the small
device grid (now a max 720px instead of 728px) and large device grid
(now a max 1140px instead of 1170px) to avoid horizontal scrollbar
problems in Firefox (likely due to box-sizing, which is why we didn't
see it in 2.x).

Similarly, we've updated the responsive ultility classes to match the
new four media query approach of the grid system. That means we've:

* Added new `.visible-xs` and `.hidden-xs` classes
* Reassigns visible and hidden classes for small, medium, and large to
matching grid system media queries
* Updates docs examples to match grid system and account for fourth
utility range

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Fixes the following:

* #9135: fourth grid tier
* #9302: undoes previous range change for small grid system, so no need
to update any docs here
* #8755: consistent grid and responsive utilities schemas)
* #9195, #9216, and #9227: no more horizontal scrollbars