Shortcodes for YouTube and iframes

Added shortcodes 2.5.0 as dependency; Earlier versions corrupted
non-ASCII characters, see
https://github.com/dmulholland/shortcodes/issues/6

The rendered elements have a `shortcode` CSS class.

The YouTube shortcode supports various ways to refer to a video:

    - `{youtube VideoID}`
    - `{youtube youtube.com or youtu.be URL}`

URLs containing an '=' should be quoted, or otherwise the shortcodes
library will parse it as "key=value" pair.

The IFrame shortcode supports the `cap` and `nocap` attributes. `cap`
indicates the required capability the user should have in order to
render the tag. If `nocap` is given, its contents are shown as a message
to users who do not have this tag; without it, the iframe is silently
hidden.

`{iframe src='https://source' cap='subscriber' nocap='Subscribe to view'}`

Merged test code + added HTML class for shortcode iframes

src/styles/base.sass

@@ -1217,3 +1217,10 @@ button, .btn
 
 	&.active
 		opacity: 1
+
+p.shortcode.nocap
+	padding: 0.6em 3em
+	font-size: .8em
+	color: $color-text-dark-primary
+	background-color: $color-background-dark
+	border-radius: 3px