`shrink-to-fit=no` is not needed anymore - Apple removed the need for it from iOS9.3 onwards

See https://www.scottohara.me/blog/2018/12/11/shrink-to-fit.html and https://github.com/h5bp/html5-boilerplate/issues/2102

* 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
placeholder.svg itself

* Change svgo configuration to retain role attribute and add/force focusable=false

- regarding `focusable=false`, see https://github.com/svg/svgo/issues/1011 (which would be the "proper" SVGO fix) and the (hacky) solution https://github.com/svg/svgo/pull/817

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.