* SVG accessibility fixes

- add `focusable="false"` to all SVGs, to stop IE from including the `<svg>` in its default focus cycle
- completely hide decorative SVGs using `aria-hidden="true"`
- add `role="img"` to meaningful/"content" SVGs, ensure they have an appropriate `<title>` and/or `aria-label`
- simplify placeholder default title

* Simplify usage of placeholder.svg in the docs

- in most cases, no need for separate/redundant text and title (now that we make text itself part of the alternative text explicitly)
- no need (expect in rare cirumstances) to describe the placeholder image (unless the look of the image is important/the actual content, as is the case with the image thumbnail class)

* Simplify example.html rewriting of <svg> to <img> code sample

- as the `alt` for the placeholder isn't really important, just add an ellipsis instead (otherwise, to be correct, we'd have to construct the whole "title+text" construct like we do now in the
place...

Edge/Mobile appears to have moved support for `-ms-viewport` behind a flag at some point, instead supporting meta viewport directives.
Removing this altogether also fixes issues in IE11 on high-dpi displays - see https://github.com/twbs/bootstrap/issues/21780

...as it always evaluates to true

The default font weight value for <strong> <br> in reboot was
set by a fixed value we now use the variable $font-weight-bolder
to let users override this value.

Dynamically set/remove `aria-modal="true"` when a modal is shown/hidden

This reverts commit 11da1601.

This is no longer needed with jekyll-toc 0.9.1.

We have /site/.eslintrc.json that covers this.

This reverts commit b442ce31.

Callouts are already excluded from ToC, but due to a limitation in jekyll-toc they are still being included.

We should revisit this if the bug is fixed later.

It's pretty small compressed; a little less than 1 KB with gzip.

Image by @mdo.

This should reduce the amount of bytes for non-2x displays.