Show "edit source" section edit links in addition to VE ones

Objective:

* Provide quick access to section edit links for both source and visual
  editing

Story:

After using this prototype of my mockup, I realized how bad my mockup
was. Hooray for prototyping!

The issues were twofold:

1. Adding down-arrows to the edit links made the page look worse, and
was sure to incite rage and panic throughout the community.
2. The menu was just too heavy. Matmarex made an observation early on
after seeing it, that it wasn't very "Vector", and while I agreed, at
the time I didn't have any better ideas.

Thank you to Matma Rex for prototyping this feature. Aparently there was
also a previous attempt (I13bbb9549). We appreciate your help.

The new design is simple.

* Section edit links look normal
* On hover or focus, the edit source link also appears next to it

To make the two links look separate, we needed a divider. To make the
divider look good we needed to add space around it. To balance the
space, we needed to add space to the brackets. To avoid changing the
view, we needed to only add space to the brackets on hover. To avoid
the text moving around, we needed to make the brackets move away from
the text, rather than the text move away from the brackets. To make
this change smooth, we needed to use transitions. To make the links not
force the heading to wrap in one state but not the other, we needed to
reserve the space, using visibility rather than display. To reserve the
space we had to use closing brackets as spacers, hiding/showing one of
them on mouse enter/leave and leaving the other always hidden. To avoid
the right bracket from getting clipped by the edge of the screen when in
expanded mode, we needed to add a bit of padding to the right side of the
section edit link top level span. To prevent the extra links from
flashing as you move your mouse down the page, we needed to wait 100ms
before showing or hiding them due to mouse enter/leave.

We use negative margins to move the brackets. Animation implemented
using CSS transitions. We bring the pipe divider in from the core
'pipe-separator' message.

To style the brackets independently we needed to wrap them in spans and
add classes to them. Change Id27555c6 in core will make the wrapping
unnecessary, but the two should still get along just fine.

Interestingly, we needed to @noflip the bracket styles because CSS
Janus flipping is triggered on UI language, but the brackets need to be
styled according to the content language.

Changes:

ve.init.mw.ViewPageTarget.css
* Add styles for extra section edit link components

ve.init.mw.ViewPageTarget.js
* Add edit source link, and make it visible when the mouse is over the
heading or either section edit link is focused

*.php
* Links to new messages

Bug: 48429
Change-Id: I4b9c47fd65a700a81c880144247fec524edff7e5