Make brackets in section edit links accessible to CSS

Each is now wrapped in <span class="mw-editsection-bracket" />.

The 'editsection-brackets' optional message, a hack added back in 2007
(43b05c00 / r23645), was removed and hardcoded again to facilitate this.

Bug: 48256
Change-Id: Id27555c6dee250eafeefa97ae8927438fd65b8d7
Merge "Make brackets in section edit links accessible to CSS"