v4 - Use "@if not $bool" expression

Rename placeholders in forms.md according to the classes specified

_alert.scss: add missing space between arguments

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docs: IE9 gradient.

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Fix #17544: Update doc for tooltips and popover about dispose method

fixes border-width issues when customizing font-size by using 1px instead of .0625rem as default -width

grammar. you're your way --> you're on your way

Adds current version to packages.

Add `vendor/bundle` to gitignore

v4 - fixed incorrect inline form class in docs

Border width variable in list group items

Added usage of $border-width variable in all _list-group.scss references

Remove Travis env vars used for defunct custom caching system

Reverts part of 42697a4e

Reverts a1c170ed

Ensure this directory is ignored so that any installed resources from
Bundler aren't handled by Git.

Use jekyll-sitemap instead of our custom solution.

Specify configBridge as relative path to source for Jekyll

Closes #17214.

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v4 - Remove dropped v3 bootstrap-theme

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This adjusts the load path for the ``configBridge.json`` file from one
that's relative to the user running the command (eg ``jekyll serve``),
to one that's relative to the ``source`` configuration setting for
Jekyll.

The result is that the user can now have a (customised) ``_config.yml``
for Jekyll anywhere on her filesystem and point to Bootstrap's
``docs`` directory to use as the ``source``.  Previously, in order to
customise it, the original ``_config.yml`` needed to be modified inside
(a forked) Bootstrap, and the ``jekyll`` command could only be run at
the root of the Bootstrap package as the original file path to
``configBridge.json`` was only valid there.

The existing behaviour is not affected.

Remove the now unused test-infra files.