Improvements to @mentions

[DJPaul]

We need to:

• Update the external JS libs as they contain relevant fixes for support in contentEditable and TinyMCE areas.
• Improve usability of mentions selection on narrow-width screens.
• Add support for the Visual editor in the WP dashboard.

[djpaul]

In 9097:

Mentions: update to latest version of ext. libs.

• Update atwho JS to rev 0f7ba07 (v0.5.1+).
• Update caret JS to rev 736a7eb (v0.2.0+).

See #5969

[djpaul]

In 9098:

Mentions: backport upstream JS for contentEditable/TinyMCE support fixes.

See #5969

[djpaul]

In 9099:

Mentions: adjustments for narrow-width displays.

For displays <= 900 pixels wide, the display name of the user is now
shown.

For displays <= 400 pixels wide, the picker UI expands to take up the
full width of the screen.

Ported from Automattic's O2 team's implementation of the At.js library.
See #5969

[djpaul]

In 9100:

Mentions: load in the dashboard's Visual editor.

Inspired by Automattic's O2 team's implementation of the At.js library
inside publishing screens.

Also updated RTL CSS missed from a previous commit.

Fixes #5969